Magician %/s (KMS FLY 2 & GMS Dawnveil)

Question about how you think Elemental Drain and Fervent Drain work for F/Ps. By the skill description, I figure ele drain works kind of like arcane aim but improves only dot damage by 15% when stacked 5 times. As for Fervent Drain, it seems to either improve each 3% improvement to 5%, totaling 25% after everything is stacked; or adds 5% to the end result of ele drain, for a total of 20%. Which would it be? Or am I completely not understanding the skill?

And about Infinity being able to increase DoT. How would that work?