The fact that they expect you to match colors visually is a good indicator that this is not an exact science. For me personally its not really about precision of what I add as it is remembering what chemical to add. I know, its only a handful of different chemicals, but then I need a grocery list if I want to remember more than 2 items. I do need to know whether to add 8 oz of something or 4 lbs, but I am too lazy to measure what I add precisely anyway, so all I need is a general idea, which the chart helps with, and I don't have to run inside to the computer. I guess I just haven't done this stuff often enough or long enough to really remember.