I have also only done a few. I used the big solid servo wheel. The I took the retract and measured how much throw it had from all the way in to all the way out. Then using the servo screw as a center point I drilled two holes directly across from each other at the same measurement as the retract throw. I hope this makes sense. Then I used those two holes to attach the pushrods. This way I was sure the servo was giving the retract the correct travel. The servo wheel also needs to on the servo so the two holes are parallel to the wing leading edge at each end of the travel.

David