Testing for CYA is really very subjective. It depends on the lighting a lot. A good way to do it is to turn your back to the sun, hold the test tube at waist level, and look straight down. A freind of mine swears she can still see the dot when I read her CYA as 60 ppm, go figure. The good news is that none of these numbers are carved in stone. They come in ranges and adjust not only for testing variations but also volume variations. A slightly higher CYA, say 70 ppm, requires a little more chlorine. A slightly lower CYA, something around 30, means the sun will deplete your chlorine faster. Once you get a feel for your pool you can pretty much manage the fluctuations in your sleep.

I prefer to maintain pH at 7.5. Fortunately, that's what my pool wants to be at, so I rarely have to adjust it. Having it toward the high range, from 7.6 to 7.8, gives you the 'wiggle room' Watermom frequently mentions, especially if you use trichlor pucks in floaters now and then. I get a lot of aeration in my pool from the dogs diving in, and we, too, replace a lot of water that they drag out on their fur every time they exit.

A 24' round with a functional water depth of 4' holds about 13,500 gallons. Your guess of replacing about 4,000 each season is close enough. With that amount of water replacement you don't have to lose sleep if your CYA level gets a little high because you pretty much dilute it over time.

One thing I would suggest is that you test your fill water (except for CYA) and record that somewhere.