When you set the linkage make sure the servo arm is 90 to the push rod. Most servos don't have a symetrical spline arangement, so you have to play with sub trim to get the arms straight, IE90 to the pushrod, not the servo case. Then the mechanical linkage, then the evil eye. If you don't set the arms at 90 degrees before the other adjustments, you end up with a asymetrical movement side to side.

Don