My edge finder is pretty crude! I hacked up an acrylic award that I got at work - cut an inside 90, attached 1/16" Al plates to that, then added wires. The offset is the thickness of the Al and that gives me a ledge to rest on the work piece. I wrote a script to work with it. Need to test the accuracy of it someday.
I've done a few double sided projects. I ended up drilling two index holes through the work and into my spoil board, then using ground 1/4" dowels to locate. That seems to work pretty well. Again, I haven't measured anything. I just finished up work on a table project. To get "woodworking" tolerances, I just used an engraving bit to mark a line about 0.010" deep along X. I used that line to align my pieces too using my 2.5x glasses. That all worked out pretty well too. I've been using a center finder bit a lot to locate the center of my work. That requires lots of light and those 2.5's again! That gets me close enough for woodworking. Honestly, I've been pleasantly surprised at how well that project has gone given that I have done multiple operations on each piece (inlay, hole drilling, etc.) and not really messed anything up and everything has come out pretty well measurement wise.
This is still in the rough....lots of work left to do...this is a combination of hand and CNC work...
Tony