the etiquette of pepper spray

i find myself spending a lot of time thinking about the moment i first get* to use my pepper spray. i run through possible scenarios in my head so i'll be ready...

...but the more i think about it, the more concerned i get. i mean--what is the protocol before spraying someone with pepper spray? do i have to warn them that i can and will mace them if they take another step, or will "i'm warning you!" be enough? i mean, i want to make sure he/she is fully aware of the possible consequences of attacking me, but at the same time i don't want to take away the element of surprise. these are the things they need to put in the instruction manuals!

*i use the word "get" here. no, i do not actually anticipate it happening, thank goodness.