DGF,
You are asking a very complex question that has no specific answer. It is an incident by incident situation, and it will change everytime the charactors change. It is also a legal nightmare for anyone giving advice. I will not tell you that it is ok to shoot in that situation, and neither will anyone else that teaches. You will just get refered to the list I gave or a similar one for the jurisdiction the person trains under.
In your description of "just getting punched" I would say that your wording takes away the fear of immediate death, and it also raises the question of if lesser force would do. Ignoring the situation and only looking at how you describe it I would say you have no legal foot to stand on concerning the use of lethal force or even drawing your gun (the reasonable man who will most likely be an anti-gun and anti-carry man in the jury).
You will not get Michael to give you clear cut lines and direction in anything he produces. If you follow The Best Defense you will see MB, MJ and RP, along with all the other experts they bring in, get very cautious and conservative on the carrying and use of weapons. This is because if they say something is ok on their show and you do it, they will end up in court defending themselves. If I tell you it is ok to do something in a given setting on this board and you do it I could very likely end up in the hot seat based on my "professional standing" in giving that advice.
All I will tell you is to know the laws of your State and local jurisdiction, know the rules of the setting you are in, stay alert, avoid trouble whenever possible, work to defuse issues, and if something does happen take a really long deep breath and think before you say anything to anyone about what happened. In a stress situation your preception of time, distance, speed and sound are distorted, and you need your adrenaline to settle down before you say anything to anyone at the scene or away from the scene about what took place. This is an area where I would prefer to error on the side of Masad Ayob, and tell law enforcement the very basics (there it the guy, he came at me with a gun, knife, club or whatever, I shot him out of fear for my life, I will cooperate fully, but I really need to speak with my attorney). Even if the attorney does nothing to change what you say, the time it takes for them to arrive clears you mind and system, and the attorney will also know everything you said about the incident. Nothing worse than your statement five minutes after the event not matching the statement an hour later after you mind clears and perspective returns.
I know this is clear as mud when you are looking for crystal. But as we start everyone of our classes "If you think you are uncertain about carrying a gun now - Wait till we get done with you today."