I think this link is relevant to the discussion http://www.icrc.org/web/eng/siteeng0.nsf/html/p0793
Prorportionality, as a concept within Just War Theory, is notoriously dodgy and hard to quanitfy, to say the least. But then again, so are all the concepts within Just War, because its the application of about 2000 years of thought to a very complex problem. However, if we use the Geneva model to discuss proportionality, I think we get a much clearer picture. As Kalshoven and Zegveld show in the above link, the commander must:
Do everything possible to make sure the target is a military one
Take all feasible steps when it comes to methods and means to minimize civilian casualities
To not undertake an attack if it may reasonably be expected to cause harm in excess of the military advantage or against the threat posed
Going by this framework, civilian casualties could be as minimized as possible, in terms of steps taken to prevent deaths, while still being excessive in terms of the importance of the target. Interestingly, these protocols bound signatory countries even if the enemy refuses to adhere to the same rules, for example by using human shields or attacking civilian targets. This is still not as quantifiable as it could be, however it does apply a certain standard which can be debated and discussed. For example, one could claim a command and communications centre is a far more valuable target than an individual member of the targeted organization. Equally, a top leader or commander within that organization is a more compelling target than a group of combatants located far away from the scenes of the fighting, and making no attempt to move towards the conflict zone (as hypotheticals).
Of course, the 'fog of war' makes this much more difficult. You are usually required to act swiftly, based on minimal or unreliable evidence, against a potentially deadly threat. Weighing the variables is going to be difficult in the extreme, in a highly stressful environment, even for experienced commanders.
Israel is, I should point out, not bound by this treaty, and neither is the USA (I believe the latter signed the treaty, but never ratified it). It is accepted by the international community at large, with over 160 signatories (with varying degrees of loyalty to the ideas expressed within), which suggests even though some countries haven't signed the treaty, it could be considered something of an international legal norm. But then we get into debates about sovereignty and the like, and I know we are struggling to stay on topic as it is.
I think, if we set aside the legal argument for the moment, there are still significant political/grand strategic advantages from adhering to a broadly accepted level of proportionality. Because of the nature of the media and the political system, this unfortunately means taking into account the views of people not in the firing line, or highly ambivalent towards the particular cause behind this conflict, but as they say, life's a bitch, then you end up in a transnational conflict situation. Well, I say that, anyway.
Claims to proportionality, for example, help legitamize the conflict itself. Especially in liberal democracies, who place heavy emphasis (rightly or wrongly) on the rule of law when it comes to their internal actions as much as their foreign policy. Suggestions that ones leaders are not concerned with proportionality suggests a lack of rule of law, of a rule of the strong which is anathema to the ethos of the aforementioned democracies. It suggests a political leadership not concerned with the rule of law, which could really undermine its claims to governance.
Furthermore, as conflict becomes less about states taking on states, with the associated nastiness of of total war, and more about taking on non-state actor engaging in asymmetrical conflict, Van Creveld's paradox comes into play. Namely that:
he who fights against the weak - and the rag-tag Iraqi militias are very weak indeed - and loses, loses. He who fights against the weak and wins also loses. To kill an opponent who is much weaker than yourself is unnecessary and therefore cruel; to let that opponent kill you is unnecessary and therefore foolish. As Vietnam and countless other cases prove, no armed force, however rich, however powerful, however advanced, however well motivated is immune to this dilemma. The end result is always disintegration and defeat...
Interestingly, and I'm not sure if Van Creveld is aware of this, Chinese military theorists may have come up with a solution to this paradox, which informs the operational and tactical level of the conflict in more detail (which I had been ignoring up until now, because of the manifest differences involved in discussing it). In Unrestricted Warfare, the infamous PLA military theory text, Qiao Liang and Wang Xiangsui take the idea of conflict without restrictions to its obvious conclusion, which is not the use of absolute force against ones enemy, as some have suspected, but the use of the full spectrum of methods available to end a conflict. Because warfare is inherently political (insert misquoted sentence from Clausewitz here), and politics is co-determinant with society at large, the book focuses on using a variety of methods, such as international law, NGOs, the media (and any other examples of soft power you can think of), hacking, terrorism, financial attacks and any other method designed to stop the enemy from having either the will or ability to pose a threat.
One of their main points is that the inceases in nonlethal weapons technology over recent years (and the book was written ten years ago) meant that a focus on causing nonlethal damage was not only possible, it could be desirable. Via maiming and temporarily wounding target combatants, one could reduce their military threat, keep the war in proportion and actually cost the enemy a lot more. Even quasi-states such as Hamas and Al-Qaeda have well developed welfare systems and payment for its 'employees'. By reducing their capacity for conflict while increasing their outgoing expenses, one would significantly undermine the organization. Morale, which is usually only hardened by enemy shows of force, could also take a hit. Equally, by reducing civilian casualties, one could focus on various soft power methods designed to decouple a subject population from the organization hiding among them. I could go off on another tangent here, but again, I'm going to try and stick to proportionality as much as possible.
Obviously this is not entirely feasible. The switch to non-lethal forces would take time and money, and one would still need to retain lethal forces for situations where their level of force is necessary. To be able to respond to all levels of conflict, from those directly on the political end of the politcal-conflict axis, and those on the opposite end, total war, one must have the means to respond to these various levels of threat.
Equally, the problem of using too little force is immediately obvious as well. The results will be a loss in confidence of the government by the people who elect it, and if responses are inffective (because this is often confused with proportionate, for some reason) then the security threat will persist, or worsen. At best, this results in a continuation of the status quo, in other words a continuation of the conflict, which is clearly suboptimal. In the very worst cases, this results in the collapse of the state as a viable social organization.
To a degree, I think drawing up a quantifiable guideline to what is or is not a proportionate response is a fool's errand. Its going to be a continual process of self-examination, tailored as a response to the percieved and actual threat of the moment. Since conflict isn't static in and of itself, at various times in the course of a conflict, and with various targets, certain responses will be acceptable and proportionate, and at other times they will not be. This is unfortunate, but it shouldn't stop people from attempting to pass judgement. I think, to use a non-hypothetical example, we can safely say that bombing a school, and killing 42 civilians, in order to remove the threat of two gunmen, is not proportionate. If the circumstances had been different, then perhaps it could have been. If the men were transporting a WMD, for example, it would be very hard to decry Israel's actions, as horrible as those deaths still would have been. Even if they had been operating a missile launcher, from a fortified position within the school which would have made other attacks impossible, could be considered legitimate (though I am reminded of the RAF captain who used to risk his life flying over factories during WWII, giving civilians enough time to escape). Equally, the deliberate targeting of policemen in Gaza, who are not taking part in the conflict (such as the Gaza chief of police, who was not even a Hamas member) does not serve any legitimate military function. Any civilians killed when attacking him, by virtue of his role, would not be proportionate.
We can also consider the overall strategic aim. As many, many people have pointed out, if Israel's goal is to stop the conflict, they are doing a piss-poor job of it. Do attacks which apparently do not consider the possibility of civilian casualties actually help, or hinder, Israel's aims? Most evidence suggests the latter. Urban warfare is bloody and protracted, and bombing campaigns againt civilians are notoriously ineffective in breaking the will of a target population. In which case, attacks which do not serve the overall strategic aim are also disproportionate. Furthermore, with sabre-rattling from Hezbollah and the possibility of international jihadist ("Al-Qaeda", if you will) infiltration into Gaza in a power gap left by a reduced Hamas, as well as the destabalizing impact of refugees on surrounding countries, suggest a strategy which is not "proportionate" in the sense that it does not deal with the problem at hand, while still resulting in lots of dead bodies.
I'm sure this makes absolutely no sense, since I have been up nearly 3 days, writing about pirates and mercenaries, of all things. I'm not really in the headspace for contemporary conflict. But if someone wants to pick this apart, tear strips from the corpse and use it to make an argument which wont, unlike this one, fall over the second I hit "submit", please do so.