The 'reasonable person' standard is the cornerstone of self-defense law in nearly every U.S. jurisdiction and most common-law countries.
Understand the question a jury will ask about every defensive action you take.
The law does not ask whether you were afraid. It asks whether a reasonable person, standing where you stood, knowing what you knew, would have been afraid enough to act as you did.
Proportional force is the second question. A shove is not answered with a knife; a slap is not answered with a shot. Your response must match the threat you actually faced, not the threat you feared might come next.
The moment the attacker is stopped, disengaged, or fleeing, your legal right to use force ends. One extra strike after the threat ends turns a lawful defense into an assault charge.