OK, I think this is how it works:
An ability has, say, 60% chance of applying a negative status effect to an enemy. The toon with that ability has 110% potency. Mulitply these two together in decimal form,
0.60 * 1.10 = 0.66 = 66% chance of applying the negative status effect.
HOWEVER, you need to factor in the target's tenacity. Suppose the target has 40% tenacity, the chance of successfully applying the negative status effect to that target (if the attack lands, of course) is:
0.66 * (1.00 - 0.40) = 0.396 = 39.6%
I'm not sure if this is correct but, having played around with this method for various toons' potencies and tenacities, there does not appear to be any particularly outlandish results.
