The rule is the 21 foot rule.

An assailant with a knife who is 21 feet away will likely stab you before you can draw, clear, and engage your weapon. That's what we were taught. With a determined person from outside of that range, you'll still likely get cut unless you connect with bone or spine with your shots.

Center mass, and don't stop shooting till they stop moving. Remove the threat.

My close encounter as a Denny's manager was 8 feet in the vestibule where the video games and phone are, and as a deputy my encounter was about 10. Luckily for all they complied immediately upon seeing my weapon. I upgraded to a .45 ACP after those encounters.