Find the center of the stringer (2 x 12), as shown above in the bridge plan diagram, and determine the amount of arch you want by tracing the arch at the bottom with a pencil as shown on the image to the left.
You should leave 15 to 18 inches from the end of the boards on the bottom cut so as to have plenty of support on the stringers for the weight of the bridge as shown on the image to the right.
One way to determine the amount of arch is to use a tack and string to make the arch or use a compass or just carefully free-hand trace the arch.