For starters, understand that Gain is the reciprocal of Proportioning Band expressed as percentage of range. Next, understand what "range" is: In SLC500 PID block, this is defined by "SETPOINT MIN" and "SETPOINT MAX" (an unfortunate misnomer). You also have to be careful to remember that all the data going into and out of this block is in 14-bit format, not the usual 15-bit. If you need PID basics, there's a great little book at simplesolvers.com. The A-B PID block is very powerful, but tricky to use.