I would imagine that someone agressivley trying to incapacitate you, or limit your ability to respond to an attack in any manner (be it response with lethal force, or simply running away) by using a less than lethal or non-lethal means, such as OC spray, could be viewed as a precursor to a physical attack that could very well be potentially lethal or cause serious injury, which might warrant the use of lethal force by the victim.
Good answer. That is/was my point. You need to be able to articulate why you believed that you'd be incapacitated by OC. That's why actually being sprayed can be beneficial.
I hope that if I were walking down the street, and someone came at me unprovoked, holding up a can of OC and telling me to give him my wallet, that would be the treated by law enforcement the same as if he came at me with a taser, or a knife, or a baseball bat, or a gun.
Just for edification, taser is also less than lethal.
At least, that's the way I see it. I have no idea if the law would see it the same way.
If you can articulate it, then you're probably okay.