Jon, Go about 3 to 4 inches around the existing hole and taper it out away from the hole. This way the glass guy will be able to build layers up starting with the smallest first and work his way out with larger pieces. After that he will then be able to fair it out quite easily..... I never liked grinding myself, but I would usually do it anyway because I know how I would want to build it back up. In your case, the glass guy is risking having to do more work then anticipated if you screw up

Good luck...I hope this helped...