hahaha, yep Max is right, evaporation leaves the goodies in the water. With a fresh fill, and provided the filter didn't get backwashed in the first day or two after adding CYA, it's all in there, other than water that's splashed out, backwash water, or overflow. So if you know your weight and water volume, the number is correct. Chlorine oxidizes CYA at a very, very slow rate, which is almost unnoticeable other than over a few months of off-season.
I agree with the experts, Kim and Marian, that 50 ppm CYA is a good idea.