Marked the rib centerlines using the method given in Ransclan. I use a sharpie pen siting in a recess cut into peice of wood such that the pen tip is .35 inches above the table. Then simply slide the penon the table, along the part edge to make a nice long straight centerline mark. Riveted the spars and ribs. My reduced headhand riveter was usefull for getting into the tight corner of the leading edge rib.