There are so many aspects of the Tueller Drill that are important to the CCW community. The intent of the drill was to show that by the time a police officer recognized a threat from an edged weapon carrier, drew his/her weapon from their holster and reacted to said threat, they would be stabbed from the distance of 21 feet. Keep in mind, the drill was performed on officers who knew it was coming, and either all of them or the majority of them failed.
This lends to the importance of not only training, but probably just as importantly, situational awareness. Most self defense shootings happen within this range, however, the threat is probably exhibiting signs of their intentions well beyond that range.
Another offshoot of the Tueller Drill is the misconception that the legal definition of being able to use deadly force is 21 feet, known by many as “The 21 foot rule”. There have been many articles in reputable publications expressing the dangers of that thought process. Obviously if a threat is far enough away, you should take evasive action. Shooting someone 100 yards away will probably become an issue for you, depending on the circumstances, however, if someone with a gun is shooting at you from 20 yards away, you would be within your rights to use force.
I’ve taken many classes for defensive carry, most of them being geared towards the laws. No hands on stuff, just use of force justification. In one class someone brought up the “21 foot rule” in which the lawyer teaching the class pointed out multiple ways in which they’d tear that person apart on the stand. The most glaring was “But the person you shot was 29 feet away”. The lawyer was pointing out that this person just taught the jury about a “rule” for engaging someone, a rule that doesn’t even exist, but a rule they admittedly broke nonetheless.
In the end, train at close distances because reality is that’s the situation you’ll be in, not because that’s where you’re legally justified, but keep in mind, there are cases where you’d be justified taking a shot from further distances. Every case will be different and the weapon will dictate your engagement and distance. And when calling 911, talking with police, testifying on the stand, remember the statement “I felt that my life was in danger so I took appropriate action” and leave it at that. Forget about mentioning a non rule that could incriminate you.