Personally I like to balance the odds as a "flavour" for the system, so in a harsh game like Mork Borg it's thematic if "normal" difficulty means 45% probability of passing the test (without modifiers); maybe you could expose a bit of stats calculations so that one could decide what "normal" difficulty means in the context of their game?
I will come up with something using your system and let you know, just for fun. Probably I will hack it a bit. Thanks for creating it!