XG2 has a tool for that under menu Window -> Elo Calculator.
It seems tested that 1 PR = ca. 33 Elo (for PRs between 0 and 10).
"Seems" because I've seen it mentioned by everyone, but didn't see any real proof/research (most informative hint I've found is that of Xavier here).

So you just enter i.e. 3PR difference as 3 x 33 = 99 elo difference (set experience to over 1000) and you get the MWC.