Skull Grasp "Increase the damage of Whirlwind by 300–400% weapon damage."
The Wastes set has "Whirlwind gains the effect of the Dust Devils rune and Dust Devils damage is increased to 2500% weapon damage."
Helm and Boots can have "Increase Whirlwind Damage by 15%." for a total skill damage multiplier of 1.3
Whirlwind naturally does "340% weapon damage."
Without skull grasp, the calculation seems to be:
Whirlwind: 340% * 1.3 = 442%
Dust Devil: 2500% * 1.3 = 3250%
How does Skull Grasp factor in?
The most prevalent answer I've found is that it factors into both WW and DD additively:
Whirlwind: (340%+400%) * 1.3 = 962%
Dust Devil: (2500%+400%) * 1.3 = 3770%
But I've also seen reports that it doesn't affect DD damage at all. Can anyone confirm?