The most effective range for FC is 7.2-7.5. When you get to the extremes like 6.8 & down and 7.9 up, chlorine loses a significant amount of "punch". This is what I was taught in my training anyway. Over the years, I've dealt with many Chemical Engineers and they generally agree with it. Once you put CYA in the mix things improve a great deal, mostly how fast the degradation of FC takes place. I am sure Richard (Chemgeek) can line out real particulars for you, but that's a simple definition.