directly from WK’s Javascript:
r = (t>=5 ? 2*Math.round(o/2) : 1*Math.round(o/2))
‘t’ is the current SRS level, and 5 is Guru. In English, this reads:
“If current SRS level is Guru or above, reduce the SRS level by 2x. If Apprentice, reduce by 1x”.
Math.round(o/2) is saying: "if you’ve made:
1-2 mistakes, x = 1 level
3-4 mistakes, x = 2 levels
So:
Making 1 or 2 mistakes on Apprentice sets you back 1 level.
Making 1 or 2 mistakes on Guru or above sets you back 2 levels.
Making 3 or 4 mistakes on Apprentice sets you back 2 levels.
Making 3 or 4 mistakes on Guru or above sets you back 4 levels.