Procedure: Drive nail into middle Of 28"". to 30""-long 2 X 8, 1/4"" in from one edge. Then-using the nail as a pivot-swing and slide a framing square so that its corner moves precisely along the 2 X 8's opposite edge. Stop the square frequently as you move it along and, with a pencil, draw a series of overlapping lines along the square's other leg as shown. These overlapping lines will describe a perfect (for this project) parabolic curve