Chuck, it isn't super critical at all. So as long as the engine isn't scrunched up against the firewall or sticking way out on the ends of the mounting rails, it will be well within the range. Much more important is to make sure that the prop thrust washer lines up with the center of the fuse (after compensating for right thrust, of course) so that the flight dynamics aren't screwed up, and so that the spinner and the cowl line up.