%w.att,? Total Damage % ? huh?

Yes, it works like that, the stacking, though, is not yet concretely confirmed. %damage surely stacks with %luk, %atk, whether or how it stacks with another %damage is not known. Words have it that it doesn't self-stack.

Total damage does not stack with total damage. The lower line overshadows the first. I tested this when I had both 6% total damage (2nd line) and 3% total damage (3rd line) at the same time, and I only observed my damage increase by 3%.