I've already answered this:
Obviously I haven't been clear. I'll try to do better.
Hague, Geneva, and Rome all say the same thing: "Thou shalt not attack a protected target such as a hospital unless it is being used for military purposes." That does not mean if it's being used for military purposes you can remove it from the map with no considerations of any sort. "Being used for military purposes" is the thing that allows you to apply the proportional proportionality analysis. If it isn't being used for a military purpose, you can't touch it for any reason at all. None, zip, zilch.
If a hospital is not being used for a military purpose but you can prove that blowing it up will somehow end the war tomorrow and save a billion lives, it's still a war crime to blow it up because you are prohibited from attacking it. You can't apply proportional proportionality because it is not being used for military purposes; it is strictly off-limits. Put a command and control facility in the basement though, and yes, now you can apply a proportional proportionality analysis to it and turn it into a crater because it's saving a billion lives.
Obviously every country on the planet would just blow it up if it was going to save a billion lives and no one would complain. But that's not the point; I'm using this invented, ridiculous example to explain what the law means and not to say I would object to saving a billion lives.
I hope that's clear: "military purposes" let's you apply proportional proportionality, it does not simply make a target fair game.