That 21 foot distance is important. It's the distance at which experts and most courts would consider a weapon like a stick to give the attacker the opportunity needed to attack you. Verbal communication is fine and I'm good at it, trust me. The problem is whether or not the attacker believes you are serious about using deadly force or is even capable of comprehending what you are saying and showing him. Think PCP. A firearm discharge is universally understood and just about impossible to not take seriously. I've had weapons fired at me and around me unexpectedly and I know that makes you a f'n believer like nothing else can. Think of the warning shot as the loudest and most serious shout to "STOP" possible.
I also disagree with the idea that a round hitting the ground two to three feet in front of the shooter can be interpreted as a shot fired at a person that is still 30 feet away.
If any shot is fired it would be very likely that the tables could be turned as any L.E./ court would consider the discharge of the gun to be aggravated assault or at the least reckless endangerment.
They will NOT agree with you in this regard.
Seek out a ranking local L.E. or D.A. & ask them & post their response.