I'm not sure I understand the saving throws for some Realm spells.

Having been recently "desourced" by an upstart province ruler that increased his population, I've decided I want revenge. The spell Death Plague seems like the ticket, but I'm not sure what the Saving Throw and Spell Resistance lines mean.

The description of the spell says that the province will lose one level, but it also allows spell resistance and saving throws. Is this just for specific individuals within the province, or everyone? If it's everyone - and you figure out the percentage of the population that will make the save - what percentage must die in order for the province level to drop?