Caught wind of this one on a language learning forum: according to recent(ish) research, when learning a new skill you should set a difficulty level that allows you to succeed in 85% of your attempts.
That’s a bit higher than I had assumed: basically you want your materials to be almost but not quite a cakewalk. 🙂
Been finding this a useful rule of thumb for things like vocab lists, learning guitar techniques, pickleball dinking drills, etc.