On page 16 it shows how to measure forward from the hatch opening.

Measure the distance from the spinner backplate to the
firewall. It should be 3-3/4” [95mm] on both sides of the
spinner backplate. Adjust the engine if needed and tighten the
screws evenly, using Threadlocker on the screws and the nuts
to secure the engine to the mount. Following the engine
manufacturer’s instructions, install the muffler to the engine