If you were to pick simple "rules of the road" for justified self-defense using lethal force, I'd say it would have to be the principle of the "Deadly Force Triangle," which requires three criteria be met: (A)bility, (O)pportunity and (J)eopardy. This is the standard promoted by the U.S. Federal Bureau Of Investigation, The Lethal Force Institute (Ayoob), and many other organizations.
The Deadly Force Triangle:- Ability -- An attacker must posess the power to kill or inflict crippling injury on the innocent;
- Opportunity -- An attacker must be capable of immediately employing that power in an attack. It's a function of many factors, including distance, time, barriers (ie, a door, wall, car between you); and
- Jeopardy -- An attacker must be acting in such a manner that a reasonable and prudent person would conclude that the attacker had manifest intent to kill or inflict crippling injury.
In one phrase, essentially, justified use of lethal force requires that the innocent be in immediate and otherwise unavoidable danger of death or grave bodily harm.
If these standards are met, if the use of lethal force is justified, then frankly it doesn't matter whether you use a knife, gun, baseball bat, car, or your bare hands ... lethal force is lethal force. And it wouldn't matter if you shot at the head, heart, pelvis, pinkie finger, or the back ... lethal force is lethal force.
Of course, that's theory. The reality is, you're on the hook at all times to justify and explain your actions to those around you. I would think you should want to have a set of standards that are generally recognized as close to morally-unassailable as it gets. Because, even if you're completely justified to the highest standard, your actions are going to be scrutinized six ways from Sunday.
For a simple run-down on the basic principles involved, check out
this useful summary put together by the Champaign County [IL] Rifle Assn, page 9, as promoted by LFI; or, the
FBI's Law Enforcement Bulletin; or get a copy of Ayoob's book, "
In The Gravest Extreme."