The important point about finding the top of the wing is to ensure the lights are level. This distance (front edge of wing to centre of lamp plinth) could vary between cars if for example the front and rear ride heights are different. It may also vary with the car loaded or unloaded. I doubt it is critical though.
Another way might be to separate the rubber plinth from the lamp, sit it on top of the wing, and use a spirit level to ensure it is level.
I think they should be exactly centred on the top of the wing side-to-side.
Before drilling try sticking them in place with double sided tape until perfectly satisfied.
