Measure the throw of the retract mechanisim. Lets say for an example the pushrod moves 1" from full up to full down and locked.

Get a servo wheel and space the two connections to that wheel 180 degrees apart and the same distance that you got from measuring, in our case, 1" apart and equidistant from center. If you have to use a blank servo wheel and drill a hole do that, dont use the holes on a stock arm that are "close enough"

That will give you perfect retract throw. MinnFlyer has a great diagram somewhere.