Sir, I respectfully disagree about your 15 foot rule. Just for grins, measure off 15 feet and get someone to time you how long it takes YOU to cover 15 feet. Then have that person time you on how fast you can draw and present your weapon.
I think police use a 21 foot rule. Even with a knife the aggressor has the chance to do some serious damage to you.
A training course I once took included the Tueller drill. I faced the target, with gun in my holster. An instructor stood 21 feet off to my left. At the signal, I was to draw and fire at the target. The instructor was to start running toward me and slap me on the back as he went by. No contest. He could have put a knife in me, or clobbered me with a club, before I finished my draw, let alone fired. Having been through that, if I ever face a threat for real, I will try not to let them get that close.