Are you talking about mounting the engine off-set on the firewall to account for the right thrust so that the prop is centered on the cowl? If so it depends on the distance from the firewall and the prop driver and the angle of the right thrust. For example, if that distance is 6 inches and the right thrust is 2 degrees then you must mount the engine mounts at 0.21 inches to the right of the firewall center (looking at the firewall from in front of the plane). The formula is:

Offset = (Firewall to prop drive distance) X sin(angle of right thrust).

cheers