One of the better discussions of the Tueller DRILL that I've seen. It's an experiment, not a rule. As others have stated, 21 feet may be a little late for a draw. When my LFI-1 class did the experiment (which you can not only reproduce in court, but maybe even get the jurors to do in the jury room), almost everyone, including a temporarily disabled person, beat 1.5 seconds. Now compare that to the time it takes you to react to a signal, draw and fire. That's more than 1.5 seconds for some of us.
Cjw3's method sounds like an excellent general response. Given an obstacle like a car or table, I might opt for running behind that if there was enough distance from my attacker to do it, meanwhile drawing. But who ever got attacked from 21 feet? Bottom line is that counting on a gun in the case of a knife attack is weak. A gun does NOT trump a knife when you're all cut up before you even get the thing out. Cjw3's response sounds like a good one for a man with a gun, and there is other training available in CQ combat.
Anyone who pulls a knife on you should be shot repeatedly until he is on the ground, but that's only if you're lucky enough to be able to do so.
[ame]https://www.youtube.com/watch?v=zI7_TTR6b1Q[/ame]