Eyballing it is close enough. You want to get it centered left and right the best you can but up and down depends on the engine, if there is a cowl, etc. on most mounts the top of the mount arms(where the engine sits on) should be the center of the firewall. As for the center of the firewall, you only need to cut as much as needed. I just drill whatever holes I need for the fuel lines, throttle. I'm sure some people do it different. Keep in mind you not going to the moon with these things. Just make sure its safe.