Apologies if this has been asked before, can't find it anywhere, including Urpriest's guide.
If you apply class levels to a creature with fractional HD, like for instance a Rat (1/4 d8 hd), do you pay 1 level for that 1/4 of an HD, or drop it? I know rats can't normally have class levels, it's just for example purposes.
I know D&D usually rounds down but both answers seem odd here.