Follow what the book says!!!... the offset is to compensate for the usual right thrust so that the Propeller shaft ends up in the middle of the cowel... In other words the engine is angled 2 or 3 degrees to the right (right thrust) and so you move the engine mount slightly to the left to keep the prop shaft centered?
Hope I did not confuse you. ( Right and left refer to the airplanes right and left as you face the same direction as the airplane.)

You do understand why right engine thrust is built into the engine mounting??