Yeah, I know what you're saying. No matter how much pinning down you do, sooner or later you have to sheet that last side...if that isn't straight, you're screwed. That last sheet is the one part that can make or break a straight wing.

Didn't mention it, but most times a piece of tri-stock works pretty well as a crutch for the trailing and leading edges. And, the accuracy of those little levels is remarkable. Also, on a constant chord wing, it doesn't even matter if the datum line is true to the centerline of the each end of the rib...just that the root and tip ribs share the exact same datum. What is quickest and most accurate is to mark one rib, then lay it over the other, push a pin through the datum line at each end of the rib and the second rib will be perfectly indexed with it's counterpart.

One last small point: Use a spot of hot-melt glue to semi-permanently attach the levels to the ribs. Makes things a little more organized. Good luck with it......if you hate guesswork, you ought to like the method.