Evaporation has no effect on CYA. Using the pucks in my pool adds about 7.5ppm per week, so it would be insanely high if I let it go. If splash out and backwashing exchanged 10% of my water every week (not even close), my CYA would raise 7.5, and drop 10% of whatever the CYA concentration was, which means it would continue to climb until it got around 75ppm.

In reality, my weekly water loss is nowhere close to 10%, so my level would just keep climbing. If you are relying on a pool store to tell you your CYA is okay, they are lying. They can't test over 100ppm, and as far as they are concerned, 100ppm is great.