I was going to use the string method but some of my turns are 37 inch radius and the board is only 32 inches at that point. The pivot point would be out in space.
i bought a couple Ribbonrail Aignment Gauges from my local train store. they just lay between the tracks. available it many different sizes. i got 24" and 28" radius ones.
Roger: Clamp or temporarily screw a piece of board on your bench to use as a pivot point.