The effect doesnt always give the full 20% per tick, more like e-20% per tick. I was casting Banof Nife ( 150 DD ) then 214 per tick for 7 ticks. BA3 made even the DD component do more damage, and tick were from 220-250 until the spell ran out. It varies from tick to tick the increase in the damage done. I got this all on my own as my Rallos Zek worshipping shaman had never touched either a giant or a dorf so I was dubious to both to start. Easy quest, if you need any finer points on this before you try, write me and Ill try to give ya a good answer.
61 Ogre Oracle --Chaotic Winds--Venerable Aggrok