I wanted to share an observation from the SLAM I am doing.
When I started, my CYA (as tested by the pool store) was 65. As the SLAM has progressed, the CYA has dropped to 40 and now 30. I understand that has probably not really happened. Even though I have been filling to replace the water lost from frequent DE backflushing, I don't think it was enough to cause that change.
It occurred to me that when I started the water was very cloudy. In a typical clear bottled water bottle, the cloudiness was very apparent. And the CYA test depends on the cloudiness of the sample obscuring the black dot (by the way my pool staore does the exact same test). So my conclusion is that when the water is cloudy, as it often is when people start a SLAM, you can't trust the CYA reading which will likely test higher than actual.