D&D General - Hit Points. Did 3.0 Or 3.5 Get it Right? | Page 11
I don't know. I consider a reasonable chance of success to be in the 75-90% range for the kind of things my character should be good at, with many things being automatic successes (generally not combat things, though). The Leverage crew is a good metric for how skilled PCs should be.
I am however OK with that success rate being achieved via some kind of meta-currency ("This is an important roll, and I'm not going to fail it.")