After learning about misattribution of effort , is there a “perfect” difficulty? Yes, in theory.
Coined by Lev Vygotsky originally, the Zone of Proximal Development is an area just outside the current skill level of the student. It is noteworthy because of the effect on learning it has: Exercises that are too easy lead to no learning, yet exercises that are too hard also lead to no learning, but to a lot of frustration. Hence the ideal exercise is one that the student can just barely accomplish on his own, or can accomplish with the help of a teacher.
By maintaining this level during 1-on-1 lessons, through constant adjustment of the difficulty level of exercises, for example, it is possible for students to make much more progress compared to a classroom lesson.
How difficult?
There are of course no fixed formulas as every student is different. However, a good rule of thumb is that the student should be able to do 60-80% of the exercises correctly on the first try.
Less than 60% means that the difficulty is too high, and instead of learning the subject, the student learns only to hate it and could even give up. More than 80% correct could mean that the exercises are too easy. Of course, this will lead to a boost in confidence, which on itself could be the goal of the lesson. Since the difficulty is too low, the student, although now self-confident, may not make as much progress as could be made with harder exercises. If there is an exam soon, this may not be acceptable.