Building the wing I used some 1/4" x 3/8" balsa for the spar, the plan said 6x8 mm and what I used is slightly larger I think but OK with me. After I had all the ribs glued between the leading and trailing edges I laid a straight edge along the spar notches and had to trim them just a tad to be straight and to fit the spar material that I used. Got it all line up and it is looking good to me.

I wanted to be sure things were straight and I could also set the dihedral of the wing by clamping it all to a six foot level.