Here is the gauge being applied to the wing for marking. I will draw a line at the aft end of the gauge on the skin with an ultrafine sharpie and finish off the skin as I show under trimming the wing skins.
I marked each skin panel at each side and on all overlaps so I would trim the skins to form a straight continuous edge from end to end of the wing. The marks were connected with a straight edge for final trimming.
This worked very well and was much easier than trying to measure and mark with a ruler in tight quarters.