If the width of the piece that keeps breaking approaches the thickness of the glass, it's just going to break later, maybe after the panel is finished and hanging. Better to make that piece wider, or put a cut in it on purpose. You could make the man's arn a little skinnier to make the piece wider, but itf that's not enough, don't try to keep it in one piece. (Unless you like constant repairs.)
Rebecca
I knew you would eventually be by and clarify what I was trying to say :-) I knew there was a rule of thumb (ie the thickness of the glass) but you told me so much that day that I went on overload and I've forgotten most of the details, I just have vague memories....