Here are some very simple gauges I made to mark the aft length of the skins (behind the rear spar) for final trimming of the skin panels prior to riveting them to the skeleton. I scaled the final length from the plans and added a little for insurance to be trimmed later if necessary. These were made from scraps of MDF. Remember that the wing is in the jig and holding a ruler steady and marking accurately while laying on the shop floor in cramped quarters was not the best working environment.

Here’s a gauge in position on the full size flap pocket on the drawings. I determined the length of the skin desired aft of the cap strip. The gauge was rabbeted so it would not be affected by the cap strip rivets.

The rabbeted leg of the gauge is held against the cap strip and the skin is marked at the aft end of the gauge.

Simple, but easy and with a high degree of repeatability.

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.