MARK IT- USING A CUSTOM COMPASS
Make a mark on the 1' x 4' boards for the center of the arc. The mark is in the middle of the board and the same distance from the top. So, if the board is exactly 12" the mark would be 6" from the top and 6" from the side.
On a scrap board mark out two locations that are the same distance apart as the radius of the arc measured above.
Find a bolt from your hardware jar that is long enough to fit through both the scrap board and the panel. Drill a hole in the panel at the marked location and in one of the marked locations on the scrap board just large enough for the bolt to fit thru. In the second location on the scrap board drill a hole just large enough for your pencil. Attach your scrap board to the panel, insert your pencil and draw the arc at the top of the panel as seen in the second picture.