4. Re-set your saw base so it's flat, then cut each 20"-wide panel in half so you have four 20 x 38" panels, each with one beveled 38" edge. With the beveled edge facing up and closest to you, draw a centerline at 18" on each panel, then make marks on the beveled edges at 8" on both sides of the centerline. Position a speed square so it pivots at the 8" mark, then rotate the speed square away from the centerline until the 22½° mark on the speed square meets the top of the beveled edge. Draw a line along the speed square as shown, then use a straightedge to extend the line to the other edge (the factory edge) of the plywood. Repeat at the other 8" mark, flipping the speed square and rotating it away from the centerline so the lines create a flat-topped triangle. Set the base of your circular saw at 40°, then cut along the angled lines (although it seems incorrect, 40° is the angle required to form a square corner when the pieces are assembled). Mark and cut the remaining three panels in the same fashion.