Just to get the straightest line possible, I measured back from the front spar rivet line the specified distances (half way between the front and rear spars) at rib 4 and at the wing tip. Then I made sure the front spar was at an 81 degree angle. Next I used a plumb line to mark a corresponding spot on the top of the 4th and last ribs. Using a chalk line, I determined the points of intersection with all the ribs in between. Finally, I dropped the plumb line at each rib (both sides of the rib) to determine my end points for each L angle section. As far as I can tell from the top, it worked well.
