I believe in the Dennis Tueller video I posted, he was asked by a police officer trainee. If they get a call there is a man with a knife threatening people, the trainee wanted to know how close to let the guy get before unloading a few magazines and hoping he hits him and not being fired and imprisoned. Even that depends on the skin color of the cop and the skin color of the knife wielding "victim".
I was lucky years ago when some nut pulled a knife that extended past his elbow on me and my family. We were parked at a McDonalds at night preparing for a along trip. I was towing a trailer so not parked real close to them, my sister and her husband was parked about 20 yards from me . The guy asked them for a cigarette likely sizing them up, they told him they didn't smoke and as he turned and headed towards my truck they told him I didn't either. He still came to my truck and asked while looking in my truck. he went back to his truck and retrieved his knife, walked slowly towards me flashing it several times. I am sure he was trying to scare me off so he could attack my sister and her husband. I pulled my Beretta 40 and waited for him to get closer. I figured I would not let him get to them, but I didn't want to start popping off rounds across the parking lot until he was close enough. After flashing the knife several times and realizing all I was doing after drawing my gun and putting it on my lap was give him a solid stare. He got nervous, backed away and drove off. He likely found a unarmed victim that night. That was Oklahoma City before they had concealed carry.
There are all kinds of confrontations. I train the best I can, and live my life. I hope I don't get the well trained knife fighter that decides to do me harm. You can't just pop someone because they might harm you, and by the time they get close enough to surprise attack you it is hard to beat. A snub nose revolver in your hand in your jacket pocket is a nice option, no draw time and you can surprise them in a hurry when you shoot through your jacket pocket. I never really practiced that, maybe this year I will take a trashed old jacket to the range.