I’ve never faced a them with a knife, but Massad Ayoob and others have written that if a they brandishing a knife and advancing on you, as here, gets within 12 feet (the figure I recall) of you, then they’re too close and you should start shooting.
Yep. It’s called The Tueller Drill and it’s taught at every law enforcement academy on Earth.
Don’t think of specific feet.
It’s properly called a reaction gap. Where the attacker can initiate before you can react. The reaction gap changes based on the situation and can be anywhere from within contact distance to more than 30ft away.