It should be based on the amount of damage a player deals to high level opponents in a round. Horse bumps would be disproportionately represented but it would, on the other hand, encourage folks to help kill rather than to kill steal. It would also, I think, fairly affect archers who spend their whole time shooting and still get few kills.
As a secondary factor, any pre-mitigation team damage dealt could reduce a players "damage score" in a round, thus reducing their chances of getting valour.