I had the same confusion. My pool is in sunlight all day and I'm putting in 2-3 Quarts a night(On sunny days. Nothing on cloudy/rainy days). My CYA=40. Now it makes sense.

A question I have is that the Best Guess Chart is based on FC. Does this mean I have to test my FC every day? I don't have enough powder(PS234) to do this all summer. I will have to buy more. I've been using the PS200 to check for CL & pH daily.