Warlock · Affliction · tier 5 · from level 30

Soul Siphon

+148 Drain Soul

3/3 · level 60 · per cast · with 3 of your Affliction effects on the target · and 2 more

Soul Siphon 123

Rank 3/3

Increases the damage done or health drained by your Drain Life, Drain Soul, and Wrack spells by 12% per each of your other Affliction effects active on the target, up to a maximum increase of 36%.

Drain Soul +148 Drain Life +89.7 Wrack +76.0

Averages, counting crits and misses, for a level 60 Warlock with a typical weapon and no other gear or buffs, against a target your level. Worked out from the game's own data, the same way as the Foresight addon.