Are you talking about the point at which the two wings meet together?
If you find that the leading and trailing edges meet but the mid points do, then sand the wing root material itself.
Typically only a little bit needs to be removed.
The trailing edge is flat, but the gap is in the center and only in the top of the wing, the bottom meets okay. I can put a flat edge across the root rib where the wings meet and they seem level.
Bryan