I came to find this forum because my CYA had risen so high over the years, I could no longer get it to a shock level that would do anything. My best guess is that my CYA was over 400 when I found this site for help. I did not get my test kit until after I had already done 2 partial drains (to keep water on the liner). Even after 2 drains, my level was still around 225, so that lead me to estimate I must have been over 400 before those drains. That is when I decided to do a full drain, rather than 4 more partial drains. How I did that is covered in this thread.
http://www.troublefreepool.com/threads/113745-New-User-Doing-first-drain-What-do-I-need-to-know
http://www.troublefreepool.com/threads/113745-New-User-Doing-first-drain-What-do-I-need-to-know