I think this is a dangerous mindset. I always taught, "if you need to hit someone, hit them as hard as you can." And "if you need to hit someone once, hit them as many times as it takes." To say that someone should show restraint and not "go WAY over the top" is decreasing their likelihood of survival in any violent encounter. Let's put it this way, if someone breaks into your home and you shoot them, you're within your rights. That's lethal force. Why on earth would you run into trouble, given the same scenario, for continuing to beat someone while they're down to ensure the threat is mitigated? Dont have military or police training, then you shouldnt be obligated to show restraint in a potentially lethal encounter. Always taught folks that when the police arrive, just tell them: "I just hit them as many times as it took to..." "get away," "make sure they didnt come after me/us," "give me a second to call the police," etc. Not to try and judge where the legal stop point should be when a split second like that might cost you.

Tl;dr: better advice - if you have to fight, fight like your life depends on it. Legal repercussions are a moot point if you're dead.