Plans next say to measure for sweep. You can't have any forward or aft sweep. It says to drop four plumb bobs two per wing and adjust wing until all four line up on a line. Actually it says within 1/2 inch is okay.
So I went to the nearby aircraft parts store and bought 4 plumb bobs; taped them to the wing; stretch a line and started adjusting. What I found was that with the wings pushed in all the way and the rear spars pushed in and touching the fuselage that all this lines up. So really I didn't do anything.
Maybe this measure is just a double check. Because I don't see any way of adjusting this unless you don't get the rear spar measured and trimmed correctly.