The design is a cobble of two others I've seen in the hobby press: one from Harold Hall's book Tool and Cutter Sharpening, and the other from one of the Village Press magazines. I decided that, although it's not necessary, I'd work to .001 as it's been a while since I needed to do that and I thought it would be fun and something to keep my skills up. So I made a dovetail slide for one of the tables that will eventually be part of a drill bit sharpening gadget. With lots of tricky clamping I match-drilled holes so everything would line up perfect. Came out great. Right on the money. Until I noticed that I'd made it a mirror image of what I needed.
As Ron Popeil (founder of Ronco, home of the Veg-O-Matic) used to say, "But wait! There's more!" I also needed four little blocks of 1/2-inch square CRS with a couple of holes in them; one threaded and one through. And were they perfect! Right down to the last .001. Match drilled, tool marks carefully removed. holes chamfered, sharp edges taken down. But then..... I discovered that there is a better way to fit the related bits together.
I salvaged the dovetail slide and, as my avitar proclaims, it's "ehh, good enough." And now I've got a set of perfect blocks. All I need now is a project that needs them.
Unless it's something simple where the part orientation makes no difference this is the reason I do a rough layout even with a felt pen showing drilled/ tapped holes, hash marked areas showing material removal etc. More complex parts get a lot more accurate layout on the surface plate. It won't stop the mistakes if you lay it out wrong, but it gives you a way to visualize the finished part before removing any metal.