If you have a set of dial calipers, it is very easy. If using carbon steel, we need to leave an edge around .030 before heat treat (or .020 if you're good!), stainless can go thinner (.015). Let's take a blade that is .125" thick, and we've used a black marker, or marking dye, to darken the whole length of the edge. Since we need an edge of .030", we take the total thickness .125" divided by two, and add .015" ( .015 + .015 = .030). Set your caliper to .078, let one of the "prongs" of the caliper ride along the blade surface, while the other "prong" tip scribes the line onto the edge. Flip the knife over, repeat for the other side. Now you have a .030" edge that is dead center. Grind to your scribed line! If you don't have a set of calipers, you can use a simple block of wood with a screw in the side of it. You turn the screw up or down, and use the screw head to scribe the line. Eyeball it one one side, flip it over and do the other. After a try or two, you'll have it dead center as well, and can eyeball your edge thickness with the thickness of a dime. But you can get dial calipers so dirt cheap, and they are invaluable in a shop.
Oh yeah, forgot about the drill bit approach. Used to use that method myself!