The firewall to rear of spinner should be 154mm. A vertical line needs to be put on the firewall 7mm to the LEFT (if you're in the airplane; RIGHT if you're looking at it from the front, upright) of center for thrust offset. The instructions call for a horizontal reference line of 57mm up from the bottom edge of the firewall (upright, looking at the front) Center your engine mount on the intersection of the vertical and horizontal lines.