From Dark and Darker Wiki
(Base * Attack1Power) + (Base * Attack2Power) + (Base * Attack3Power) / [Sum of all Speeds]
So for a 20 base damage Castillon Dagger it would be (20 * 1) + (20 * 1) + (20 * 1.5) / (0.579 + 0.131 + 0.569 + 0.180 + 0.943 + 0.200 + 1.061) = 19.11 DPS