After leveling the wing and aligning the skin with the trailing edge of the rear wing spar (and clamping in place there), you need to set the hole in the rear wing spar fitting at exactly 19" from the wing station point.
Picture 1 shows the whole setup. The drywall T-square is aligned with the hole in the wing.
Another scale is clamped with the 19" mark aligned with the edge of the T-square (photo 2). Now I can tweak the hole in the rear spar fitting (left-center of picture 1) over until it aligns with the end of the second scale (call it a "ruler" or a "yard stick", but technically they're called "scales").
Photo 3 shows the trailing edge of the upper aft skin clamped only to the rear spar. At this point everything looks properly aligned.
