It sounds like the offset is already marked on the box...assuming its correct for your motors length. When you look at the front of the box, you should see that its tilted starboard to account for the engine torque. The offset is typically measured so that the prop ends up being in the center of the planes thrust line and engine cowl.

Is the thrust line of your engine in the center of the engines mounts? If not, try and get an engine mount template from the engine manufacturer which will show the absolute center and horizontal/vertical center...then you just line everything up, mark, and drill.