The "ideal" number for CYA really depends on the pool. We generally recommend that people start at about 40 ppm, and then see how much chlorine they use. This works for most folks--but there are some, like me, who tend to run theirs higher. My pool is in full Louisiana sun from sunup to sundown, high humidity, high bather load, and I have to keep my CYa at 80-90 ppm in order to keep chlorine levels where they should be. If my CYA gets any lower, my chlorine consumption increases dramatically. Having a CYA of 90 lets me use a 1.4 gallon jug of bleach every 2 days or so to maintain 8 ppm of chlorine. However, my pool is a bit of an exception. Until you "learn" your pool's habits, I would start with about 40 ppm of CYA and see how it goes--if you're still having trouble keeping chlorine in the pool (due to sun, not to algae) then you could bump it up a little and see if that helps.

Janet