Unless things have changed since I built my RV-8 wings, there are two layers of material at the spar bars, and there may be a little bit of air between those two layers. I used a couple of C clamps to squeeze things together before measuring the thickness, to better simulate the way things would be once the bolts were tightened. I found that Van's numbers were pretty close.
I made my spacers from 5/16" nuts, 0.003" thicker than my spar:
http://www.kilohotel.com/rv8/article...21022173215335