For those interested in the quantitative reduction in chlorine's effectiveness at different levels of CYA, see this chart and this graph. These were done at 80F, but there isn't that much difference at the hot tub temps of 104F.

The rough basic rule (applicable when CYA is larger than FC) is that at a pH of 7.5 CYA reduces the disinfecting/oxidizing form of chlorine (HOCl) by a factor of 75% of the CYA level. So, a CYA of 30 ppm reduces chlorine's effectiveness (compared to no CYA) by a factor of 0.75*30=22.5 so that the killing form of chlorine is reduced to 1/22.5th of its concentration compared to no CYA.

Richard