I've personally always advised my students to increase difficulty once they've reached 87.5% accuracy.

(I'm just amused of all the % thrown around I this thread)

Exactly. You need to be in the zone of proximate development or learning isn’t very efficient. I remember as a teen I had a fixed mindset and would get 100% on every calculus test. The problem became that I liked getting a perfect so much that ended up actively avoiding harder material and just sticking with what I was taught in school. I was afraid of posing interesting problems to my self out of fear that I wouldn’t be able to answer them.

This is a dangerous and corrosive mindset that did a lot of harm to me and other people I know. It took me maybe until 25 to actually start to accept my lack of knowledge and incompetence and start to do something with it. I sometimes wonder what my life would be like if I had the emotional maturity to challenge myself at a young age instead of pursuing destructive perfectionism.