Why not devide this task in two? I used a shorter measuring stick made of stable 3/4" strip of plywood, that could reach the 1/2 way mark of the frame. 1 for the front section to center area and 1 for the rear of the frame to center.
Drill out all necessary hole locations in the stick. Then I compared left to right holes by just flipping the plywood measuring stick left to right on each side. Repeated this on the opposite end of the frame. Fairly accurate and no sag in measurements. Hope this can help.
