See this post by Richard. There are charts that show FC half-life versus CYA level.
This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...
www.troublefreepool.com
In my own pool I have measured FC losses of less than 1ppm over a 24 hour period in the hot AZ sun because I use high CYA (90ppm is my target) and low FC levels (3ppm or less) so about a 3% FC/CYA ratio. I can get away with that because I carefully manage my water chemistry, use borates and treat for phosphates (PO4 < 150ppb). It is more time consuming on my part but I happen to have time to spare...