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:
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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