Jeff McMahan has a nice article clarifying the notion of 'proportionality' and applying it to the current Gaza conflict.
I just want to add a quick supplementary thought, which we might call the Cosmopolitan Civilian Test. Begin from the cosmopolitan idea that the moral status of an innocent civilian does not depend on their nationality. Alas, actual human beings have a psychological tendency towards various forms of nationalism and tribalism, counting some innocent people for more than others. In particular, many responding to the current conflict seem relatively unconcerned by the Palestinian civilian casualties. One obvious way to correct for this bias is to imagine that these killed civilians were not Palestianians, but some other nationality -- Israelis, say.
Hence the Cosmpolitan Civilian Test: Would these civilian casualties be considered "proportionate" if the civilians in question were of a different nationality? Suppose Hamas somehow managed to use Israeli rather than Palestinian civilians as human shields. Would it be "proportionate", or worth the cost, to blow through a human shield of innocent Israelis in order to get at Hamas? Are Israel's military goals in the current conflict worth killing 1000+ of their own civilians for? If not, that seems to indicate that they are not worth killing innocent Palestinians for either.
(And if you reject Cosmopolitanism, it's worth bearing in mind Bob Goodin's observation -- from 'What is so special about our fellow countrymen?' -- that nations often have even stronger negative duties to innocent foreigners than to their own citizens, constraining what they can do to them.)