Roughly speaking, the chlorine amount needed to prevent algae growth is proportional to the Cyanuric Acid (CYA) level. That is, the FC/CYA ratio roughly determines the amount of "active" chlorine available. If you have twice as much CYA in the water, you need twice as much FC to have the same level of disinfection and algae prevention capability.

The breakdown of chlorine from sunlight is proportional to the amount of active chlorine. So a certain percentage breaks down so, in theory, at the same FC/CYA ratios the percentage of chlorine loss should be the same which means the absolute loss should be higher at higher FC and CYA levels.

However, Janet (aylad) and others have found that keeping a higher CYA level, especially around 70-80 ppm, has even the absolute FC loss be lower in spite of keeping a consistent FC/CYA ratio (i.e. having a higher absolute FC level). Mark (mas985) did an experiment in this thread that showed that at a CYA of 45 ppm, the chlorine loss per day was around 50% while at 80 ppm CYA the chlorine loss was only 15%. So even accounting for a higher absolute FC level, one would have a lower absolute FC loss at higher CYA levels.

For example, if one had the FC be around 7.5% of the CYA level which is close to Ben's "Min." column in the Best Guess CYA chart, then 3.4 ppm FC at 45 ppm CYA would lose half or around 1.7 ppm while 6 ppm FC at 80 ppm CYA would lose 15% or around 0.9 ppm -- around half the absolute FC loss per day.

I never figured out exactly why this effect is occurring. There is a protection effect (from UV in sunlight) from CYA for the lower depths of water and this effect is non-linear (exponential) with respect to distance (depth). So if the chlorine is depleted near the surface, then there can be a strong CYA "shielding" effect that is non-linear. In addition, there is some absolute chlorine loss independent of sunlight so that becomes a larger percentage of chlorine loss at lower FC and CYA levels. Say that there is a 0.7 ppm FC loss due to non-sunlight factors, then this would represent a 21% loss at 45 ppm CYA while it represents a 12% loss at 80 ppm CYA. This doesn't explain the entire effect, but combined with the CYA "shielding" effect, it might.

Richard