Originally Posted by Paco

The common law rule is that you have a duty to retreat if you are able to do so safely. Most jurisdictions have this (it cuts down on street fights).
Most importantly, the force that is used in self-defense has to be reasonable (and unfortunately that includes proportional) vis-a-vis the threat faced. That is, you can't go and shoot somebody in the face b/c he pinched your girl's ass.
Once you are at home and somebody comes in with the threat of deadly force you are entitled to use deadly force (since you can't retreat from your home!).
If you are in a business that gets you into touchy situations I'd advise looking at your state's penal code. While I practice law at the fed. level you'd be well advised to ask a buddy (over a beer no charge) what are CALIFORNIA'S laws regarding the same. And just b/c you are not criminally liable for an assault and battery doesn't mean you couldn't be slapped with a CIVIL cause of action for the same. There's an old book by Carl Brown called "The Law and Martial Arts" if you're interested in the topic you should take a look at it.