For what its worth, when I rebuild forks, I refit into the bike and then undo the top nuts and allow the forks to slide down the sanctions.
I then use a syringe tube to take a measurement of my base oil setting when the forks are sitting down on thier sanctions springs etc all in.
Then during a race weekend I can reset to my baseline easily, and then I add or subtract xx ml from / to each fork leg. Based on tube diameter and approximation for springs etc, I pretty much know where I am mm wise.
Good advice this!