Closest I can find is on p4, under "Measuring Distances", which states that you use the closest part of the base when measuring distances. I'd say that if the closest part of the base is in terrain (i.e. any of it is), then the model gets the cover save (and would have to roll a difficult terrain test to get there or move out). This is supported by the "Units in Cover" example diagram on p19, where two of the Orks in the crater are only partially in the terrain but are shown to have a cover save.
|