Here Mr. Badman, hold my tape measure so I can check and see if you are 21 feet away. ???
21 ft is just a guide too .. if you are slower than avg (and 1/2 would be) then maybe a 30 ft rule is more appropriate for you.
I go by the 100 yard rule on my land .... (my land is only 100 yds !)
When I took my Instructors Class, we did the Tueller Drill. There were 8 in my class ,average age of the guys.. 50. There were 4 active or retired LEO's, 3 retired military, all active shooters in one discipline or another.... and me. We had a B27 target about 15 feet in front of us.
We had our firearms holstered and when we were told , we were to draw, and fire 3 shots center mass. When we drew to fire someone behind us ran the other way and dropped a tent stake each time he heard a shot. Most shot low on 1 or missed, missed center mass or grazed on 2 and hit center mass on 3. The average distance for the shot 3 was 35 feet.... well farther that the 21. Had it been an assailant, most would have been stabbed long before getting a stopping center mass shot on target.
Its an eye opening drill, try it, you will be surprised at the results..