I just checked my builder's log. We used a 2.5" pitch between stitch lines, adjusted slightly for clearance around spars and other internal obstructions. We also stitched all ribs with the exception of the root, end and the portion of the wing containing the fuel tanks. We have a Rotax 914, so the 2.5" spacing between parallel vertical lines was the selected for higher airspeeds. I think our PolyFiber book is out at the hanger, so I can't double check, but that's what I recall. Frankly, the more important aspect is to achieve the nearly vertical alignment of the twine rather than using the top or bottom surfaces as your reference for deteremining which you use for the pitch distance. Note that, yes the top is a curved surface, but the 'chord' between the 2 fabric puncture points is the obviously the same top and bottom using template method shown in my previous post. If you stitch every available rib with this (2.5") distance you should be fine. Two other factors....our EAA Tech advisor has looked at this (he's a pretty detail oriented guy) and was satisfied. We also know the local FAA FSDO rep and he happened by the hanger a few weeks ago while the wing tips were off, and pretty much looked over the bird and was pretty impressed with how we've built it. It's still your call as the builder, but this was our process.

BobRS & Kitfox Kath
N104Y, SS7, Rotax 914
Alb, NM