I don't have a clue and doubt such statistics exist. I know its not that difficult, a woman now my wife was 50/50 for a couple days for head trauma from a violent assault, In my youth I've seen people beat to death, and my comment in the Zimmerman case is based upon head violently meeting concrete.
However, I do not think a person has to be being beaten to death to use deadly force to stop a violent assault by a stranger of unknown intentions. Rather, a person can use deadly force if that is what is necessary to stop the assault.
First, no person has to allow him/herself to be violently beaten even if not in a deadly way. Second, there is no manner to know when it would become deadly or permanently disabling - whether deliberate or not. I've never claimed that TM was trying to kill ZG. Maybe he was. Maybe he wasn't. No way for Zimmerman to know nor does it matter if he did.
IF Zimmerman's story of him sticking to the sidewalk to try to see where Martin ran off to, so he could tell the police he had called where Martin approximately is, is no wrongdoing at all on his part. Had he not done so, his call to the police would have just become a prank call by Zimmerman in effect, a total waste of police time.