The opening should start next to the main spar and go toward the traling edge the length of the servo. Make sure the opeinging leaves clearance so that the sides of the servo case are not rubbing anywhere. I usually wrap the servo case with 3 or 4 layers of masking tape to make sure I am fitting and drilling the mount screws with this clearance locked in. Then remove the tape for final assembly, doing this there is no issue of vibration transmitting and affecting the servo.