So it really boils down to our understanding of what the laws of your state are, and how thoses laws will be interpreted by the jury. You must be able to present your actions as being within those legal limit as judged by the cultural standards of the jury. What works in Arizona or Colorado may be harder to sell to a jury in New Jersey or Illinois.
As a partially disabled 57 yr old vet living in Colorado my response to the original post is very simple. I believe the attacker to have the intent and physical ability to inflict lethal bodily harm to myself and or others at this moment in time. I am in fear of losing my life if I do not utilize possibly lethal force to protect myself and others.
BANG!! BANG!! BANG!!!
That said, I've had three instances where the use of lethal force would have been justified but the attackers fled when they realized they had brought knives or baseball bats to a gunfight. I'd like to thank them for fleeing so I didn't have to present my justification for the use of lethal force.
Timidity in the face of violence is unbecoming to God's image. - Jeff Cooper