Check out the post linked above @SNAKEMAN , though I think the forums formatting of my original response may of mislead folks into taking what I said a wee bit off the deep end.
Defender has 50 tenacity. You have 5 potency = Defender has 45% chance to resist.
You have 10 potency, defender has 40% chance to resist etc.
until you get to 35 potency, when the defender has a 15% chance to resist any more potency past that is useless against THAT target, because they can't have a lower resist chance than 15%
So if you have 50% potency, they still have a 15% chance to resist. Or if you have 100% potency, or 100000% (Mind you, this has NOTHING to do with the trigger chance on effects, either)