I was getting inconsistent angle of incidence measurements using a simple level with a strip of ply attached, so I made the level a bit more elaborate.
It became apparent that the wing skins level out aft of the rear spar rivet line, so I literally had to check about 100 times to be sure I was measuring in the right spots and parallel to the airflow. Without doing all this it's hard to be sure both wings are at equal angle of incidence and if they aren't there will be a consistent roll tendency to correct with the ailerons that would lead to unnecessary drag.
Once completely sure the wings were at the same incidence I began to drill for the bolts, ensuring the wing spar tooling holes would be completely consumed by the 5/16" hole.