I'd rather avoid changing N by level because
a) it doesn't make sense for you to take more damage from the same enemies upon leveling up, and
b) resistance ratings are more ambigous when the multiplicative resistance they convert to varies by level (even though they can easily show the conversion for you)
Not just N by level, but N by level differences. If you gain a level and the monster stays the same level, then there are character level differences involved that might lower the damage they do to you even further. The rating will show you how much % resistance you have against monsters of your level. Lower level monsters will do less damage than that, and higher level monsters will do more. There will be some character level normalization curve that adds in penalties for larger differences in player/monster levels, but that's another math adventure for another time.
Earth or somewhere like it.