I think you’re calculating percent change in S, not absolute change in S. That may be intentional, but I think absolute change in S would be more useful for PSG purposes.
PSG = (S_Recall - S) x R
With percent change in S, it’s basically the exact same sort as difficulty_asc because that’s exactly the function of D in the algorithm (the sim results show it’s not the exact same, but pretty damn close).
Taken from https://github.com/open-spaced-repetition/fsrs4anki/wiki/The-Algorithm:
- The larger the value of D, the smaller the SInc value. This means that the increase in memory stability for difficult material is smaller than for easy material.
With absolute change in S, you’d be maximizing the actual time between reviews. That makes more sense to me.