I am making strides everyday in figuring this maybe the old school way. I started trying to cut 12" x 12" x .75 simple carving stuff using super glue/activator and tape.. I cut a grid on my waste board but it seemed impossible to get it to lay/square, so I took a piece of plywood cut perfectly square with table saw sled exactly 12" long ,2" wide, and marked my waste board in the exactly, matched the 12" X 2" in center again exactly and screwed it down. So I have a dead center piece 12" wide screwed to the waste board so I can butt up a 12" exactly center by waste board two inches from the front lip of the machine. So I take my 12" x 12'' piece of wood and flush it up right and left to the piece screwed into the waste board, smooth as silk, { I cut all my wood on the table saw sled with a stop on it set at 12",I am bit Anal

}.I then take a couple of bar clamps and clamp it the piece of wood screwed to you guessed it the waste board. The meal rods on the bar clamps rest right on the sides of the wood, hand tighten both sides, there is no way for it to move.,
I took the point of the V bit measured over 6" front and back / side to side, set my bit on the wood with paper,, set zero on all three axis. Raised Z up to attach my sweepy bottom on it. Opened up my program in UGS, turned my router on with shop vac, already had ear plugs in and head phones on, and hit run!
It ran perfectly centered up, came back to dead center again after finishing.
I can raise it up in Z a little more and put another piece in, or shut it down for the next time it is already centered X and Y, just set the bit on the paper and take off again. Over and over and over again,, simple ,quick a dead center !
Have a very blessed day,
Ray