Tubing Benders

Hellow
I purchased 2 sets of tubing benders (1/4 & 3/8) the same type that the HVAC pneumatic control installers used. They are not the traditional Rigid or Imperial but they are "T" shaped with a sliding wheel. To use them you place the tubing between the top of the "T" and the sliding wheel and brake the tubing around the wheel with your hand. That much I know. What I would like to know is how you figure gain and offsets there are NO markings of any type on eather tool.
These were a flea market purchase so there is no instruction manual .
I have worked as a construction electrician but now I am a plant maintenance electrician and I do not see any HVAC control fitters any more and from what I see it is all but gone as a trade.
Let me say that the only instrumentation type fitters that I have seen with these benders are the HVAC control fitters.
I placed this thread in the automation section in the hope that someone with pneumatic controls installation experience will see it and answer it.
THANKS Slim