The traditional self-defense rule is essentially this
As far as I'm aware this is the law (unchanged from common law) everywhere, possibly phrased in slightly different ways. Differences arise when it comes to the use of
deadly force in self-defense. As killing people is a big deal, traditionally this required multiple extra things:
1. A reasonable belief that someone is about to cause death, serious injury, or one or two other very violent crimes to your person
2. A reasonable belief that ONLY deadly force could prevent this; if you could have shot your aggressor in the foot or head just as easily the law doesn't like it if you go for headshots.
3. You can't have instigated the conflict yourself
(the "initial aggressor rule")4. You don't have the option of attaining safety walking away without using deadly force (the "duty to retreat").
Note that rule 4 never applied to your house; since "a man's house is his castle" it was treated as the place where you should feel safest, so you had no duty to retreat out of it (the "Castle Doctrine"). What "Stand Your Ground laws" do is extend this everywhere: in states with SYG laws, you only need to go with the first three rules to use deadly force. Here's how all this looks in the Model Penal Code: