I've been doing this for over a year and I still have trouble deciding on my CYA level. The black spot on the bottom becomes invisible, then I think I have added too much, then I take out a little and can't decide if I really see a shadow or not. Do other people have this problem? Maybe I am a slow learner, or just old and dumb.
If 10 experienced pool owners on the forum each tested the same water for CYA, do you think they would all get the same reading? If not, do you think the readings would all be with in 5 of each other? How far apart would you guess the highest and lowest reading would be if we could do this experiment?