That is, indeed, an excellent question. Let me just add what I think and you can consider it FWIW.
You cannot pull a knife in a fist fight, you can defend yourself against serious bodily harm.
So, how to tell the difference? If you are just losing the fight and are suffering bodily injury that doesn't mean you are justified. If you become defenseless, like on the ground in a fetal position and you are getting kicked, then things begin to look alot different and a knife becomes a reasonable defense against an assault that could inflict serious harm.
Remember, a knife can wound to a degree. That means stabbing, slicing at the assailants hands and arms or feet is different than stabbing at the heart or chest.
You've heard of defensive wounds when a victim tries to fend off an attackers' knife and receives cuts about their hands and arms, if you assailant picks up an object and starts to use it on you then it is reasonable that you use your knife to injure his weapon hand in order to stop the attack. These objects could be a bat, a stick, a bottle, or a knife.
I hope that's helpful. Everything in this area of law is about what is reasonable. You don't want to be the person who escalates the level of force. You will be judged on your reaction to force, not your superior skill or choice of weapon to end the attack.