What do you mean "fail chance"? You mean like when a popup shows up that says "Not Enough Arousal" or whatever the stat in question is? If so, then those are based on a flat threshold. Like, for a certain event to progress, you always need 15 Arousal.
If you're talking about things like the butt/boob groping events, I'm pretty sure those also run off a threshold system too, but instead you have a given amount of actions you can do based off your Arousal/Guilt/Trust, and some actions count as more actions. Like, spanking counts as more than just groping.