From what you described, I would say you handled it extremely well and kept our wits and your cool. I live in Miami and we were rated the worst road rage in the country. If someone approached my car in and aggressive manor and started smashing at my window, they would be staring down the muzzle of my gun. I would not be "asking" him to back away. It would probably be more like "back the F up". Any sudden movement that appears to be reaching for a weapon would probably be grounds for pulling the trigger. The aggressor is outside of the vehicle where he has unrestricted movement. He has already blocked your route of escape and made verbal threats against you.
I know these things happen very quickly, but getting 911 on the phone is the most important thing. Since all 911 calls are recorded, it will serve as evidence for what transpired.
I'm glad you were not injured and I'm glad you did not have to shoot anyone. I strongly suggest you file a civil suit in top of the criminal charges for damage to our car. The next lesson that hot head learns may be his last!
The ATF should be a convenience store instead of a government agency!