I think that we can all agree that 20 yards is too much and that 21 feet is damn close. Unfortunately the only guidance that I would give is from common law, the elements of the defense of self-defense;
1) Be innocent of instigation. 2) Be in reasonable fear of bodily harm. 3) Use sufficient force only to deliver oneself from evil. 4) Attempt to withdraw.
Know that local ordinances may be more liberal or restrictive than this.
I-ANAL and a coward.