If you're using a large affiliate that pays the cap (27% RB), you can approximate it pretty closely with 0.27 * MGR. It will actually be a little high over time though because Full Tilt deducts some things from your MGR that PT can't see (overlays in tournaments, freerolls, store purchases and deposit fees are the big ones). But you can use it to give you some idea of what RB you're due. Just don't expect it to be completely perfect.