Why BBB Method?

[brittmer]

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.

Evaporation does help because then I need to add new water to the pool.
I test cya myself and most years I can keep it around 60-85 fairly easy.