[ska]

That definitely works better than a timer, but still has to be adjusted for the water quality in your house.

But it also assumes you go fill-to-empty. In my limited experience they are often refilled from partially empty, whenever convenient. If you are doing this on paper, maybe you should count output instead of input? More work though.

[lancesells]

Didn't think of water quality. I'll have to see if I can test it. I live in Brooklyn in a superfund site so it'll be interesting to see.