I make a pair of thin ply plates, about 6 inches by two inches, then undo the servo linkages, then clamp the ply plates to the wing so that the aileron/elevator/rudder is centered between the two plates. Then I tighten the clamps a bit to hold it there, then I set the servo linkage so that it is attached without pulling or pushing on the surface.

This makes the aileron (elevator or rudder) on the same flat plane as a flat bottom wing (vert or hor stab), or centered on a semi or symmetrical wing form. At least close enough to make them centered so that both sides are the same, which I believe is what is desirable. If they deflect slightly down, well, that changes lift (as flaps) or up (as spoilers) but that too can be adjusted out. It at least makes them the same and flyable.

Some may disagree with this, but it has worked for me so far. Sharing any other ways of doing this would certainly be appreciated.

CGr.