tenacity - potency = chance to resist (with a minimum of 15%)
a 90% potency character will get resisted just as often as a 65% potency character when attacking a 80% tenacity character. You'll need atleast 15% more tenacity than the attacker's potency in order to have a better chance of resisting.
Long story short, tenacity sucks. I do prefer tenacity to suck though, if tenacity was a viable option we would see alot more *resisted* pop-up, wich would make arena less fun imo.