Also found this in the FCOM 4:--
OPERATION RULES CONCERNING PREDICTIONS
— The pilot must keep the flight plan data properly updated during the flight in order to get accurate and meaningful predictions. — The pilot should rely on the navigation display for short-term predictions. It indicates what the aircraft will do under the currently engaged modes (selected or managed). — The pilot should rely on the MCDU for long-term predictions when managed guidance is active or about to be re-engaged.
ie if your active waypoint is close by and your groundspeed won't change much then you can use the ND for guidance. If you are climbing or descending into a changing G/S then use the MCDU.
