In the early 80’s Dennis Tueller began to look at knives vs guns and found some surprising statistics. And so the Tueller Drill (https://en.wikipedia.org/wiki/Tueller_Drill) came to be and is now taught to all LE. In fact, some years back it was posited that the 21 foot rule should be increased to 30 feet. I am in favor of this change since at 21+ feet you can still be severely injured or killed by an assailant with an edged weapon. And since we are not equipped with laser rangefinder eyes (at least most of us) I’d rather err on the side of safety. So for me, 30 feet it is. There are a multitude of Tueller Drill videos on YouTube, so check them out.