Oh yeah I think you're right. I was thinking about it backwards, because it'd still only be a 15% increase compared to the damage you're doing, even though it's a greater DEX increase.
i.e.
600 DEX (740 Damage) - HE - 690 DEX (850 Damage) +110
1000 DEX (1600 Damage) - HE - 1150 DEX (1840 Damage) +240
Still an awesome skill that I wish NIN could use.