Would be useful like for example getting the right ammount of water for the quantity of purifier you currently have.
Could be a part of advanced inventory menu, but having it ask every time you want to unload anything liquid with regular unload would get boring quickly (and isn’t all that useful).
I don’t see much of a reason to do integrate that, to be honest? To get the exact amount of liquid X for recipe Y? Just grab the big container with liquid X and store it somewhere near where you can reach it every time you want to craft Y?
I’d like to hear a scenario where it actually would come in handy to have the EXACT amounts of liquids ready instead of most of the liquid and just choosing how much you want to produce (batch crafting ftw)