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While preparing to drill the bottom wing skin to the ribs, I wanted to be sure that the center line of the ribs were straight.
When doing top wing skin I had used a metal strip with holes drilled in a straightline to be able to verify straightness.
The improved version uses a 1/8" acrylic strip about 2 inches wide with a line drawn to verify the center line of the rib flange is straight.
In addtion, two red lines were added that would give me a warning if a drilled hole might get too close to the edge of the flange.
Video provides a few more details.