Gee I learned the two elements were Perception and Proximity (NC Justice Department CCW Instr Class)
1st - Perception. A reasonable person must perceive an eminent threat of death or bodily harm (includes rape).
2nd - The person threating must be in close enought proximity to carry out the preceived threat.
Example - I couldn't blow someone away if he is standing 50 yards from me threating me with a knife. I would meet test #1 but not #2. However, change that knife to a rifle then I meet both tests and could defend myself.
Of course after meeting both tests then you get into the matter of retreat/avoidence and this really depends on State/Local Laws, etc.
NRA Life, Lodge 1201-UOSSS
"Si vis Pacem Para Bellum"