Of all the "classes" I was required to go through during my military career, these types were some of my favorites. As a youngster (back when I was 10 feet tall and bulletproof)

this was some of most eye opening training I received. In my later years, as one of the instructors, we always stressed distance, which in turn gives you options/decisions..... and when distance wasn't an option, "Disarm" is the objective.
Two lessons that were always drilled into use where:
1. Deadly Force: That force which an individual would know, or should know, would cause serious bodily harm or death.
2. Three circumstances that are required to be present in order the justify the use of deadly force:
A: Opportunity (Does the offender have the opportunity to cause you harm?)
B: Capability (Does the offender show/posses the capability to cause you harm?)
C: Intent (Does the offender show intent to cause you harm?)
For the average (not combat trained) person, the "21 feet rule" simply doesn't apply, and has been debunked many times. On the other hand, with a "trained" individual, that same "21 feet rule" was for many years, a "golden rule". Over time tactics taught have changed to compensate....which is why you see police officers often draw their sidearm immediately upon exiting their vehicle in recent times.