I would not let anyone get closer than 10 feet in a SHTF times. I have seen gangs in concerts here in Miami use knifes against other gangs from about 20 feet and it is fast. This was 30 years ago when I saw this. By the time it was over there were three down on the floor and many with cuts all over. I saw over 50 police officers get in and arrest many of them.
Knifes are very fast if they are close to you. So the rule is do not let anyone come close to you.