Ken,
I don't think you are able to cut tempered glass. If you did, you only got lucky. The nature of tempered glass doesn't allow it to be altered. So how is it "cut" in the first place?
Before it's actually tempered. I asked my glass guy if there is any way to cut it. Well there is. First, you have to anneal it....untemper if you will. That is a big process in itself. Then there's the question of how to temper again. It sounds like too big of a project so I never even asked about that part.
I have seen floating around on the internet something about laser cutting tempered glass.
Bottom line, I wouldn't count on being able to cut it with any traditional means.