Let's not over think this. The "21 foot rule" was originally made up to demonstrate that if you didn't carry C1, Mr. Bad Guy would be on you before you could draw, rack the slide, and engage. It also concedes 1) Your first shot would be as soon as you clear your holster/pocket. And, 2) You'll likely get hurt either way, but if you're not in C1, things could really get ugly.
I don't have the link but a couple of years ago I saw a video in which a woman, who was relatively quick, dressed as most of us do when we are carrying.
She carried at 3:00 under a light unzipped jacket.
Her "attacker" was equipped with a stage knife, probably rubber.
From 21 ft., he charged her and got her ever single time.
Three times.
Only one of the times, as I recall, did she get her weapon almost into position but got hit before she could fire. Even if she had, she would have missed and at the very best shot him in the foot.
Her failure was not that she did not have time to rack the slide....her failure was that she never cleared the holster two times out of three and the third time never got the weapon into position.
Memory can be tricky, but my memory is that he knocked her to the floor a couple of times with a manoeuvre similar to a body block.
My memory could be faulty on that last point.
But the real eye opener was that this was a staged attack with the "victim" well aware of what was about to take place and she still failed to beat him.
Unlike others here, I have no advice and will not preach any methodology.
My only advice is to think about it.
I am sure the link is still out there somewhere. If anyone has it, please post it.