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: www.spaco.org/bk.htm and click on the link "A Self Centering
Scriber for stock up to about 2" wide".

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