For minimal throw, place the control rod in the hole closest to the center of the servo arm and in the outermost hole on the control horn. For maximum throw, place the control rod in the farthest hole on the servo arm and innermost hole on the control arm. Any combination in-between will give you throws, well, in-between! You can play around with the placements and see how the surfaces respond.
I like to use the farthest arms out on the servo just for the mechanical advantage, this means using larger horns but I believe that it is more worth it especially under higher powered operations. Had a Gremlin flutter because I had the holes too close to the servo and too close to the elevon. Not really enough force to hold everything in place