The 85% Rule for Learning - Scott H Young
We don't learn very well if the tasks are so easy that we get it perfectly right every time, or so hard that we're constantly failing. While it's not the same for every task, this suggests that learning is more effective if you can succeed 80-85% of the time.
The 85% rule suggests growth will be maximized when we practice tasks we can succeed at roughly four-fifths of the time.
If you succeed in every attempt, you probably don’t have the difficulty high enough to improve. If you fail most of the time, you will likely make more progress if you start picking smaller, more manageable challenges.