I had a CYA level prior to covering my pool over 200 past winter. I did a partial drain a week ago and now I'm at 80 or so. Still much too high. I am doing another partial drain now. My pool is starting to get a little algae so Im putting a gallon or so of chlorine in daily and will slam it when I get down to 50 or below cya . Doesn't seem to any accurate way to determine how much water to drain down to get me to 50 or less.