You say you are measuring down the wing every 3 inches - Are you measuring the leading edge, the trailing edge, or both? Have you checked the wing is square to the centerline of the fuse by measuring from the tail to each wingtip? I'd still suspect a twisted wing, but another possibility is the rudder not being square and inducing some yaw at all times. You would need to offset that with aileron also. Or the horizontal stabilizer is not parallel with the wing.