Years ago I also did some experimentation w/ midi-device and SRS ( https://www.youtube.com/watch?v=a6tvHMvF8Mo ), where the focus was on ear-training rather than score-learning.
Clef seems to be a pretty strong attempt at a high difficulty UX. I've created an account and will be giving it a go. Wishing you luck, and thanks for sharing.
What a great application of spaced-repetition!
How do you determine _when_ to prompt again?
Thanks! It's determined via the FSRS algorithm. It starts out with the average retention curve for flashcards, but then adjusts to you as it learns your patterns. In the future, as I have more data, the initial curve can be tuned for piano specifically (since muscle memory degrades differently than plain text).