In the summoning portal skaadi’s tab says she does 30 damage per turn per stack. In the ascend tab going from skill level 7 to 8 it says she does 25 damage per stack per turn.
The damage per turn works like other DoTs in that they benefit greatly from attack power. Maxed Skadi has 696 attack, while yours is at 548. Once your Skadi is level 80 the DoT will be the same, because they will have the same attack power. From the DoT guide: