Here's a link to a neat little tool you can make. You just lay in on top of a bar, let's say 3/4" square, and draw it along the surface, holding the legs firmly against the sides of the stock. Even works on tapered stock. I saw it at a blacksmith conference several years ago and I give credit to the guy who showed it to us, Tal Harris.
Go to:
If you make one, and the scriber isn't centered perfectly, just reverse it and scribe again. The center will be half way between the two lines you make.
Pete Stanaitis