60% to 65%.
Humans are weird. Our intuitive (and more importantly
emotive) perception of probability and events is
not particularly accurate. You see this problem, for example, in films that actually feature an equal number of male and female characters. Historically, female characters have been less common, and
much less central, than male characters, which sets an expectation. As a result, when people* see a film that consciously works to ensure an equal balance of male and female characters, both in terms of quantity and of narrative importance, most audience members,
even women, will often perceive an overabundance or preponderance of women even though there isn't one.
When it comes to bad events vs good events,
truly fair and equal probability does not
feel fair and equal. It almost always
feels incredibly biased toward failure! In order to prevent players from feeling like they fail all the time, you need to have success rates around 65%, maybe a bit lower. And yes, that means people
feel like they fail more often than not unless they succeed about twice as often as they fail! This is not a rational part of human psychology, but it's mostly pointless to try to fight against it.
*I'm fairly sure this comes from studying Western cultures specifically, so it might be different for non-Western cultures.
So, if you choose 50%, you are saying you want your PC to succeed at the task half the time and fail half the time, making success rewarding and failure a bit painful.
See, I find the latter half of this statement nonsensical. The
chance of failure is completely different from the
pain of failure or
rewards of success. You can have a 5% chance to do something completely pointless, like tossing exactly four heads in a row (p=0.0625), and a 95% chance to do something that will get you your heart's desire, or something that if you fail you'll curse that day for the rest of your life. An outcome being unlikely does not, and
cannot, make the outcome more
rewarding or the lack of that outcome more
painful. It may, sometimes, make the uncertainty more (or less) dramatic, but it can't change whether one
cares about the result or not.