Gord, you are the father of knowledge.
Yes low FAC is an indication of reserve after oxidation. The lower the reserve (after dose) the higher the biological loading if you were dosing a consistant qaantity of chlorine or bromine. The best way would be use an ORP probe/mV system and measure the mV of the waste to be treated. Measuring that with pH would then enable you to dose chlorine based on a setpoint and maintain a consistant reserve. I wouldn't use chlorine with sewerage, rather Chlorine dioxide but it would depend on what levels of sanitisation you were trying to achieve.
Why use chorine at all in a head
? I just don't see the point.