it all depends on the percision of the bit and how steady you are.

im not 100% sure, but im thinking that if you put some of that browish tape over where youll cut it may help hold the glass together for some support. thats what they do with windows in cuba where theres lots of wind of the ocean. they make a taped x acros the window and it helps to stop it from cracking (or at least from a full-on shatter) covering the entire area with hot glue gun will achieve the same thing imaging and give you a big thicer substance to align with. (just an idea-ive never tried)

try with another piece of thin glass around the house.