I figure my scale would be:
d4 = you technically can bend, but you've had no training and have no knack for it.
d6 = you're pretty alright at this.
d8 = you've had some serious training or you're a natural.
d10 = you're a master at this. you've been practicing for years.
d12 = you are one of the most powerful people on the planet in this skill and in general. you could count your peers on one hand, if you have any at all. or you're the avatar channeling the avatar state to their utmost.
But even then, the Avatar state can be used to do things beyond _any_ human capacity from time to time, …