If you're into trig... the amount of engine offset from the vertical centerline of the firewall is ....

offset = distance from the firewall to the back of the prop x tan (degrees of firewall right offset)

example:

with 3 degrees of firewall right offset and the distance from firewall to back of the prop being 6 inches

0.314 inches = 6 inches x tan(3 degrees) or (0.0525077)

5/16 " is very close to 0.314 inches ... so offset the engine 5/16 of an inch to the right of vertical centerline looking at firewall from the nose of the plane.

Hope this helps...

Larry