I don’t think there’s an official source because the FAQ and Guide are way out of date. But that’s definitely how it works. (The best official source is the code.)
It goes down by:
ceil(WA/2) * M
Where WA is the number of wrong answers given for that item (meaning or reading) during the same review session and M is 1 if the item started in Apprentice and 2 if the item started at Guru or higher.
For example:
| # of Wrong Answers | Decrease in SRS if Apprentice | Decrease in SRS if Guru+ |
|---|---|---|
| 1 | 1 | 2 |
| 2 | 1 | 2 |
| 3 | 2 | 4 |
| 4 | 2 | 4 |
| 5 | 3 | 6 |