Richard:
How is it that "a pool with 10 ppm Free Chlorine (FC) and 30 ppm CYA has the equivalent disinfecting chlorine level of 0.45 ppm FC with no CYA"? Is there a typo in that? That is a ratio of over 20 times. Looking at Ben's "best guess" chart below I would have thought that 10 ppm FC at 30 CYA would have been equivalent to somewhere around 5 ppm FC at no CYA.

Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm

..Bill