Demon Sac - SuccyFollow

I've got a question concerning the Succy DS. Does the +15% shadow dmg is applied on the base spell dmg or on the total dmg (including the +spell dmg)? Example: Let’s say I cast a SB that does 1000 dmg, 500 from the base spell dmg and 500 from the +spell dmg (fictive numbers here). The boost from DS will be 500 * 15% + 500 or 1000 * 15%? I believe it’s on the total dmg but I would like to be sure before respeccing.

Second question: How does it scales with other boosting talents like SM? Is it 10% * 15% (26.5% total) or it simply stacks 10% + 15% (25% total)?

Thx

Percent bonuses to spell damage, like succubus sacrifice, of the effect from CoS, go after everything else, and are all multiplied together.