Actually, the hole should be drilled perpendicular to the top of the aileron. This puts the control horn pivot point closer to the hinge line. It is also the easiest why of drilling as all you need to do is place the aileron upside down flat on the drill press table and go to it. Perpendicular to the center line will also be OK but perpendicular to the bottom of the aileron is gonna cause a problem. It's simple to sand the "A" nut at the angle you need so it sits flat against the bottom of the aileron but after sanding you have to hold the "A" nut tight to the surface while screwing the bolt in from the top or it will chew up the aileron.