Here's the scenario:
- Your PC is "good" at a task (whatever "good" means to you), not great, but definitely above average.
- You are trying a "difficult" task (whatever "difficult" means to you).
What chance of success is perfect for you, the "sweet spot" if you will; where the enjoyment of success meets the risk of failure?
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.
You have two votes for your response, in case you want a slightly wider range.