Can you use the center of the work as your Zero point? If you make a jig with a short enter dowel that the clock face would drop onto, you can get repeatable results.
Make your jig out of something with a sharp corner so you can always put it in the same spot on your machine. Mount a dowel to it that does not stick up higher than your clock face. The center of the dowel will be your X0 and Y0 locations.
This next part only works if you have limit switches and have a repeatable machine home position. GRBL will let you store multiple workpiece Zero locations. The G54 through G59 commands change these locations. G54 is the workspace (material zero) the machine defaults to after a power cycle or reset. You can use one of the others as your Zero point so you can come back to this project whenever you want.
I did a forum post about my investigations into this a while back. Here is the link.
EDIT: I saw @FrankWiebenga post pretty much the same thing as I was writing this up.