you just add the exact right amount of water needed, depending on your medium and size/maturity of your plant, climate, etc. that way all the moisture (or a good part of it) is utilized by the plant or evaporates. it has alot to do with experience. im sure theres a formula but i flunked algebra 3 bloody times. so if youre like me youll just have to drown a few girls to figure it out!