Professional sports often divide athletes into separate divisions to ensure the competition is fair. It would be ludicrous to put a boxer from the heavy weight class in the ring with a bantam weight boxer.
 
When you are forced to use deadly force the legal system looks at the same thing. If someone attacks you with a bull whip and you respond with a firearm you have used disproportional force. To continue with the same line of thinking as above, your attacker is a bantam weight athlete, and you are responding as a heavy weight athlete.
 
That was a conceptual discussion. The literal example is you responded to a non-deadly force threat with deadly force. You have violated the proportionality aspect of self-defense and negated your self-defense justification for the use of force.
 
In a self-defense event you must respond with proportional force only. However, if the attacker is using deadly force the proportionality consideration is irrelevant. You may use any level of force at your disposal. If your attacker is using a .22 caliber pistol and you respond with a 50 BMG, you both are using deadly force. The prosecutor cannot use the proportionality argument to say your 50 BMG is a disproportional response. Both calibers are deadly.
 
Another aspect to proportionality a disparity of size. If your attacker is significantly more capable, such as a 90 pound, 80-year-old woman being attacked by a 200 pound 25-year-old male, there is a disparity of force. This gives the woman grounds to use lethal force in her defense.
 
If a single person is being attacked by multiple attackers, the defender can use deadly force to counter the disproportional force being applied.
 
Likewise, if an altercation, not involving weapons other than fists, results in one party being no longer able to defend but the other person continues the attack, the attack now meets the threshold of being deadly. Then an elevated level of force can be used by the person who is unable to defend otherwise.
 
When looking at a response, what is deadly force is somewhat ambiguous. For example, a baseball bat is normally not considered a deadly weapon. Yet, it can be used in a deadly manner. A person’s hands (no matter how many times you hear the Hollywood “these hands are registered as deadly weapons” line) are not considered deadly weapons. However, if the attacker grabs the victim by the throat and begins strangling the hand are now deadly weapons.
 
Punching someone with a closed fist is still not deadly force, if it were, most professional boxing matches would end quickly and with a death. Here another aspect comes in, duration. The professional boxing matches have a referee who stops the fight when a participant is getting repeatedly punched and unable to continue defending against the punches.

As you can see proportionality is a murky pond. This is why the best advice is to not get into an altercation in the first place. If it is unavoidable, be careful to not use more force than required to stop the threat. Consider law enforcement responses. They start off with de-escalation first. If that fails, they increase the level of response until the threat has been neutralized. You should do the same.
 
Only you can make the judgement call at the time of the event. If you carry pepper spray but use a pistol first, you (actually your lawyer) must defend that decision.
 
Remember, I am not a lawyer and none of this should be taken as legal advice. This is training only.

In order to use lethal force, the person using that force must be in imminent danger. The Blacks Law Dictionary (https://thelawdictionary.org/imminent-danger/) defines imminent danger as:
 
In relation to homicide in self-defense, this term means immediate danger, such as must be Instantly met, such as cannot be guarded against It calling for the assistance of others or the protection of the law. U. S. v. Outerbridge,27 Fed. Cas. 390; State v. West, 45 La. Ann. 14, 12 South. 7; State v. Smith, 43 Or. 109, 71 Pac. 973. Or, as otherwise defined, such an appearance of threatened and impending injury as would put a reasonable and prudent man to his instant defense.State v. Fontenot, 50 La. Ann. 537, 23 South. 034. 09 Am. St. Rep. 455; Shorter v.People, 2 N. Y. 201, 51 Am. Dec. 280.
 
I see the words; “immediate danger,” “instantly met,” and “threatened and impending injury.” I also see “cannot be guarded against it by calling for the assistance of others or the protection of the law.”
 
Massad Ayoob teaches about the AOJ triad in his MAG-40 training and Andrew F. Branca discusses it in his book, The Law of Self Defense. This triad consists of ability, opportunity, and jeopardy.
 
The elements of the triad are considered when determining if self-defense actions meet the immediacy test.
 
The first element is ability. Does the threat have the ability to cause you harm? Of the three legs of the traid, this one is least likely to negate the self-defense argument. Having the ability to harm someone else is just about universal. The degree of harm can certainly be a point of discusson, but that is a different leg of the triad.
 
The next element is opportunity. Does the threat have the opportunity to cause you harm? Someone seeing you walking on the ground floor of a mall and begins threatening you with a knife from the second or third floor doesn’t have the opportunity to carry out the treat immediately. That is a very simplistic example. Branca’s The Law of Self Defense gives several examples to include case law from various states in how opportunity is adjudicated.
 
The third element is jeopardy. Does the threat, with ability and opportunity, show intent to cause harm to you or another innocent party? That places the innocent party in jeopardy.
 
An example in Branca’s book is a security guard in a bank. He has the ability and opportunity to do harm but, unless you are a threat yourself, has no intent to cause harm. Thus, no jeopardy.
 
Once these three elements exist, deadly force can be used. That is, as long as it is proportional to the threat.