BAD Minnflyer,

Sorry couldn't resist. I have a small tool that scribes the center line on the surface for you I believe it was made/sold by carl goldberg as part of their hinge sloting kit. You place on the surface and twist it so the two tabs on the ends touch the top and bottom of the surface then apply some pressure and run it along the surface and the center line is scribed takes about 2 seconds.

If I had some artisic talent and all the cool graphics editing programs Minnflyer has I'd draw youa new diagram But instead you are stuck with my descriptio n