Shooting someone, or even drawing on someone in response to getting slapped is extremely likely to result in sucessful prosecution of the person with the gun.
There are two CHL cases that I am aware of where someone has shot someone in response to an unarmed assault.
In the one case I am more familiar with, the CHL holder's injuries included severe concussion and permanent hearing loss. Yet he was STILL criminally prosecuted and might have been convicted had he not hired a good lawyer. What eventually made up the juror's minds is not absolutely clear, but the fact that the defender was quite small and the attacker was quite large probably played an important part--autopsy photographs of the attacker showing his impressive size and musculature and a "Born to Kill" tattoo were probably the clincher.
In the second case, the CHL holder took what was described as a "severe beating".
BOTH cases definitely resulted in criminal prosecution in spite of the fact that CHL holder was severely injured.
There are certainly cases where drawing on, or possibly even shooting an unarmed person who is attacking you in public** makes good sense, but it is important to understand that if you shoot in such a situation, criminal prosecution is a certainty. If you have sustained no injuries, and there is no obvious disparity of force (multiple attackers to one defender, large, fit attacker vs small, disabled defender, large male vs small female) then the prosecution is going to be successful.
Remember that you and your lawyer will have to convince a jury that what you did was reasonable and immediately necessary to prevent serious injury or death. That is the ONLY legal justification for using deadly force.
IMO, getting slapped is not justification for drawing on a person, and it is certainly not justification for shooting. Here's my reasoning. If a person has the chance to hit you in the face and they choose to make that blow an open-handed slap rather than a punch, that would almost certainly be viewed by a reasonable person as evidence that severe injury was not the goal of the slapper. If you can't convince a jury that the person was intent on severe injury then you won't be able to convince them that it was legal for you to threaten or use deadly force.
At that point, the thing to do is to withdraw from the confrontation and call the police. With witnessess, you are going to be able to get a criminal and very likely a civil judgement against the slapper. If you choose to pull a gun, the tables are turned and now you're the one likely to be facing criminal charges and civil prosecution.
So, i should just resolve myself to be slapped by some punk?
Your gun is there to defend your life, NOT your honor. You have a legal right not to be slapped--if you are, you can have the slapper prosecuted. But since there is no threat of severe injury or death, that "right not to be slapped" doesn't extend to a justification for you to use or threaten deadly force.
**
Note that shooting an unarmed attacker who has broken into your own home is a totally different situation in legal terms. Since this is primarily about CHL specific shootings, my comments relate to things happening in public vs shooting a home invader.