Here is a thought experiment that I want to run through. Let's say my lawn is very dry and I water it. My guess is that the moisture balance would not be "full". How do I know physically when I am "full"? From there I think balancing will be easier, but what defines full is a question that I have.
I am also wondering why I see 9" as a default for lawn root depth. If you go down 9" on my lawn you are into decomposed granite with zero roots.