Damage mitigation is done at a flat rate - whatever damage would have potentially been done is reduced by a set factor. If the mitigation is greater than the total foe's damage, then typically only 1 hp of damage will be done. The damage mitigation effects stack additively with mitigations from other augments - these include Bastion, and Fortitude for physical damage; Awareness and Apotropaism for magick damage; and Autonomy for both types of damage.
The attack boosts stack multiplicatively with other Augments that increase Strength and Magick. In Dark Arisen the total cumulative increase in Strength and/or Magick from Augments is limited to +80% (i.e. 1.8 times).
Augment effects are active from dusk till dawn (9pm till 5am) for both Gransys and Bitterblack Isle. This is slightly shorter than the period which pawns keep their lanterns lit (8pm till 6am).