You can also google more info on the math behind the oval method such as oval height vs oval width = x distance between nails and x string length.
Where? Google isn't as smart as you think. For a 19' span you would probably place the nails a couple inches in from each side to get a foot or two rise. I can see how you would do it by guessing, but I don't see anything that gives you the math.
I haven't done this but instead of playing around with the nail locations and string length can't you do it in one shot? If you want a 19' span, snap a line on your plywood and make two marks 19' apart, then make a center mark at 9'6". Say you want a 2' height, measure 90 degrees with your framing square at your center point, make a mark at 2'. Put a nail on that mark, hook your tape to it then measure 9'6" from the nail until you hit your center line that you snapped. Put a nail right there then do the same thing to the other side. Then you have your nail locations for drawing your ellipse, right?
