Here's a sketch of what I'm talking about. Obviously, the arcs I drew for the edge radius is very exaggerated, but you should be able to get the point.

In this case the scribe lines actually intersect - so you can connect the intersection points and get the actual center line (shown in red.)

Depending how you set up your scribe, it's also possible that the two scribe lines don't intersect. If they're close enough you can simply find the middle of the "gap" between the two for the center line, or readjust the scribe and do it again so they do intersect.

http://www.tgrmedia.com/i.ashx?w=800...ap_730X550.jpg