I used the old ladder trick when I install my Cowl. I just take a step ladder, rest the gear on the the ladder with the plane in an upright position. Place the engine on the firewall, and then put the cowl on. Move the engine around on the firewall until the engine is centered with the cowl............Mark the bolt holes on the firewall, or remove the cowl, without moving the engine. Presto.....you now have an engine that is perfectly centered with the cowl, and no measuring was required. Good Luck.