I personally would have my weapon unholstered and in a shooting grip long before the danger got within 21 feet. Enter the kill circle and you’re already looking down a .45-caliber smokestack.
Your move, tough guy.
Every situation develops differently and, therefore, every situation requires a different solution. If “shoot” is your default solution, you may find yourself sitting in a prison cell. I keep the 21 foot rule in mind as the situation develops. I know what 21 feet means but, as a civilian, kill is not my default solution. If they enter the 21 foot circle running at me as fast as they can with a shiny knife gleaming in their hand, there’s not a lot of thinking that needs to be done but I have yet to have a situation develop like that. If you’re being ambushed, they may be within 3 feet of you before you even realize that there is a situation. That’s why I believe situational awareness is what saves the day and the 21 foot rule is one aspect of that situational awareness.