You are now assuming a crackhead or guy hopped up on meth thinks rationally, are you serious here? Within 21 feet is considered "danger close" for someone, even with a knife to be able to get to you before you get a shot off...you will not be able to dictate the terms of this engagement
Where I live NC I am applying for my CCW and they better be real close to be a threat or the prosecutor will think otherwise. I mean you have to be in imminent danger.