I think you're making this harder than it really is...all your son should do is test the water, see what the result is, and then add bleach to get it back over 5 ppm. If he needs to add 1 ppm, he should add 2 cups. If he needs to add 2 ppm, he should add about 4 cups. If he needs to add 3 ppm, he needs to add about 6 cups. These are estimates, but they will work to make it as simple as possible.

If you stop using dichlor, your CYA will eventually decrease via splashout or water lost during filter cleanings--but it's going to be slow. Plan on keeping 5 ppm of chlorine in the pool for awhile...

Janet

Janet