The board going across the top of the steel beam puts the outside rails at exactly 3-1/4" from the center of the beam. I had a lot of difficulty getting Bulkhead C located as called for in the plans, by using the vertical upright method. This way places the outside rails of the fuselage exactly where they are supposed to be. If you have a center mark on the board it also centers the outside edges of Bulkhead C. The 1/2" "C" channels locate it at top and bottom so this makes it accurately located, if you built Bulkhead C and the "C" channels accurately.