Matt- I don't know if this has been brought up yet or not... My experience with model aircraft has proven that extended distances on paper are never right. This is because the size of the paper changes with temperature and moisture content. The reason that the rib height and width are correct is that it's a small distance, and a negligible error. The distance from the two spars is a greater distance, and the error is amplified. That's my theory. I wish I could tell you a good method of correcting for it, but that's beyond me : ) Sorry -Nick Stolley