Not only that, but that chart has been proven to be wrong. Mark did an experiment described in
this post that showed that at 45 ppm CYA, the FC dropped by about 50% while at 80 ppm CYA the FC dropped by around 15%, but these both started at the same FC. This means that even scaling the FC proportionately with the CYA to keep the same disinfecting chlorine level, a higher CYA near 80 ppm will lose less absolute FC than a lower CYA near 45 ppm. This was a surprising result, but one that was seen by other forum members as well. For whatever reason, Mark is now seeing a lower loss of around 25% of the FC per day at 50 ppm CYA.
As Jason points out, there are many factors, but I think that there is some sort of additional protection effect at higher CYA levels that more than makes up for proportionately higher FC levels. So in very sunny areas, running at a higher CYA closer to 80 ppm makes some sense IF you are very careful to not let the FC drop too low since fighting algae at a higher CYA level requires a much higher FC level. This is also the reason why the CYA recommendation is higher for SWG pools since it lets one lower the SWG on-time (since less absolute FC generation is needed) resulting in a slower pH rise over time.
Even with no sunlight, there is a chlorine demand. In my own pool, it seems to be somewhere around 0.7 ppm FC per day or thereabouts and that's at a typical FC of around 4 ppm with 30 ppm CYA. I suspect that this amount is a percentage only of the active chlorine concentration, so roughly the FC/CYA ratio, but that's speculation on my part.
Overall, I would say that roughly speaking at 40-50 ppm CYA one loses 25-50% of the FC per day while at 70-80 ppm CYA one loses 15-30% of the FC per day. At 100 ppm CYA, it may be 10-20% of the FC per day. So Greg, your loss does seem to be in the wild-guess range I've just proposed.
Richard