Actually, the "30 - 40" ppm range on your CYA test makes me suspect that the guy may actually know what he's doing, and that your test results are reasonably accurate. The reason is, none of the CYA test methods available for home or store use are more accurate than that. When some one gets, or gives, a reading of CYA = 33.1 ppm . . . I KNOW they are bogus. Perhaps, unknowingly bogus, but still bogus! It's unusual to encounter pool store staff who actually UNDERSTAND the test methods they use. You may have found an exception.

Of course, if he DOES understand, and he told you that 300 ppb phosphates means you MUST do something, means he may be smart, but dishonest! 3000 ppb would mean you might have a problem you'd want to fix; 300ppb means almost nothing!

You can continue to increase your CYA level if you like. Just read the Best Guess page so you understand what's going on. Chlorine USAGE is pretty much the same, regardless of which bracket on the Best Guess chart you're in. But at higher levels of CYA you have to add more chlorine at a time -- but you can do so less often.