I needed to reinstall the wood rib jigs to check what length I should make these ribs, but the sub panel needs to be held off the wood sub panel jig by .040...the thickness of the flanges to which the sub panel is clecoed. So, I made up some .040 spacers, taped them in place, and also drilled a couple holes to allow clamping the aft rib jig.

Rib jigs in place.

So, the ribs need to be 14 9/16" long.