I have a 25K gallon in-ground plaster pool, in Phoenix, AZ. CYA is over 150+ per Leslie's. I have ordered a test kit and extra reagents. I will pump out 40%-50% of pool volume to lower CYA. Since I am in Phoenix, will full-on sun until at least 2pm or so each day, I am thinking a CYA level of 60ppm will be good. Of course, it goes without saying that I'm dumping the Tri-Chlor pucks and shock, and going all BBB as soon as the test kit arrives.