I’ve always figured the 21’ rule applied to me or anyone being attacked. 21 feet is your weapon is in your hand and primed and ready to use too me. Prepare, weapon down and ready. Try to calm the issue down. Escalation by the attacker is Go time. Shoot to stop, how ever many it takes. IMO
The Tueller Drill teaches that an attacker can cover 21 feet in 1.5 seconds, which is longer than most people can effectively draw from concealment.
If an armed belligerent is within about 21 feet, you should draw and issue commands to drop the weapon. If he instead advances toward you, you should shoot to stop. Even if you shoot an attacker at 21 feet, if he's hyped up sufficiently, he can still close the distance and stab you before he drops, so once you start shooting, you should keep shooting until he drops.