Okay try to picture this. Start with a 72" 2x6. Draw a center line with a tri-square across the width of the board (at 36 inches doing the math). Draw subsequent lines 6 inches away from center on both sides of center. The 6th line from center should be at the end of the board. Now, take the end of the board, start at the top draw 5 marks every 1/2 inch. Do this on both ends of the 2x6. The bottom (5th) mark will be 3-inches down from the top. This will provide a 3 inch reflex at 72 inches. So, if you draw a horizontal line from your first 1/2-inch mark to your 1st (6 inches from center) 6 inch mark you will have an intersecting point. Do this for the 2nd 1/2 inch mark on the edge to your 2nd (12 inches from center) mark and so on. Do this on both ends of the board. Where each of the lines intersect, you will connect the dots forming your arc. Cut out the arc.
Hope I explained it in words well enough. If I didn't, I'll post a picture of one of mine.