Standard caveat that I am not a lawyer and not providing legal advice applies...
I don't know (and really don't care) what precipated the incident. You handled it pretty darn well; nobody got hurt and nobody went to jail. Things I might have done differently, were I in that situation, might include driving to a police -- or even better, fire, since they're more likely to be there -- station, while calling it in; pulling out of the lot without engaging the guy; driving somewhere other than my regular destinations (do you want some yahoo like that to know where you work/live/exercise?); confronting the guy, maybe with an apology to blow him off his script.
When might you have been justified in employing force against him? Let's remember a basic definition for a defense of justification in the case of self defense: You may use the force reasonably necessary to safely resolve the situation. Some of the factors that go into determining reasonably include imminence of the attack, your reasonable belief that the attacker can carry out an implied threat (ability), and your options other than using force - especially lethal force (preclusion). Let's try to analyze this encounter along the way, and see if we can pinpoint when you might be able to articulate a justification for what level of force... (Yeah, folks, this is what articulation comes down to. Practice it often, because when you need it, you won't be thinking straight.)
Some sort of traffic incident happened, and the other driver followed you to your destination. Force presented to you? Nothing really, he's just following you. Might be time to change your destination, but you probably can't justify pulling a Secret Service ramming reversal and escape yet. Or winging shots out your window as you drive on... (Yeah, hyperbole, quite confident it wouldn't have entered your mind to do either.)
You're both in the lot, and you realize he's followed you there. Has he blocked you in, or can you drive on? Unless he's blocking you in, he still hasn't really done anything other than follow you. Not much justification for using force yet, huh? If he's blocked you in, maybe... but you might want to wait to be able to better articulate an ill intent towards you. After all, he might just have been stopped behind you looking for a space (I know, unlikely, perhaps, but not unreasonable) or he might have been trying to tell you about a flat tire or other issue with your car (again, unlikely, but not unreasonable).
He exits his car. Things are escalating. You can read and interpret his body language tolerably well; there's a difference between someone coming up to warn you about your car and someone ticked off at you. I'd encourage calling the cops now. Takes time to get through, takes time for cops to get there might want to get that clock started. Also, especially with speakerphones on a cell phone, it's easy to give a play by play to dispatch. But you're probably still not really justified in using much force. Your options are limited (you're in your car, you may or may not be armed...) since your mobility is limited. You're probably not justified in running him down just to get away without really making a solid case for why you feared significant bodily harm... is weapon visible? is he alone? His shooting gesture isn't the same as brandishing a real gun; nobody's been killed by a pointer finger gun yet, as far as I know... But... Pair that shooting gesture with a motion towards a waist band as if to access a gun? Maybe. A lot depends on how you describe that motion. A visible gun? Certainly has escalated things. So... are you precluded still? Depends. Can you drive away? Can you drive away with only property damage? (which is more valuable a life or somebody's planters?) Can your car power through what might be in front of or alongside you? All of that figures into whether or not you could articulate either driving over or at him, or shooting him -- because the way you apply lethal force (force likely to cause serious bodily injury or death) doesn't matter.
So... as you can see, not a yes or no, bright line point... and I'll bet the whole encounter was probably less than a couple of minutes once you were in the lot.
Let's look at a variable... what if you exited the vehicle? Your escape options may increase (you can't drive through a parking garage wall, but you can run along it) as well as your force options... but you become much more vulnerable, too. Your car may not be cover (it won't stop bullets in any meaningful way, unless you happen to have armored your car!), but it's still a heck of a barrier for a person to overcome in grabbing or striking you. Now you have a new range of articulations to deal with. And I guarantee that the first thing an opposing attorney will ask is going to be along the lines of "if you were so afraid of my client, why did you get out of your car?"