it has to do with tourque. the farther from the center of the servo you make the connection the less tourque that the servo can put out to the control surface. it is like using a wrench to remove a bolt the farther you grab out on the wrench you convert the extra distance moved in to torque but when you have the bolt spinning the wrench the torque is converted into movement. using holes closer to the center also provides finer degrees of control. I usually like to start in the middle of both though and start moving holes one at a time. If shorter throw I start moving first the hole on the servo one then the hole on the control arm one hole next and reverse the order if I have to lengthen the throw