Just a thought, if you can place a pot on a scale, say a bathroom type scale, you would be able to get a visual on water use more or less. The digital glass top kind, they switch off automatically so all you'll have to do, is lift the pot slightly, put it down again, and bob's ya uncle..... Dunno, perhaps useful in your scenario.
A mechanical one might do it too, but less accurate, at least In the cheaper end of scales.
Yes, I've thought about that. I might pick one plant and test that idea. Of course, different plant will consume at different rates, but the vilume of water needed will always be the same for any given size of pot. Maybe this:
Water a pot till it has some runoff. wait maybe a day, them weight the pot. That would give you the saturated weight and tell you how much water you used. Reduce that amount by, say 10-15 % and that may be the max watering. I'll give that idea a test!