I personally believe it's all RNG ( i have hit over 120k damage on a boss and got one epic item only and from that same boss got 3 epic items from 70k damage ) but this threshold list is still helpful.
Originally Posted by Allwyn
I heard the thresholds were updated... or i heard something funny?
Cthulu's post is dated : 08-23-2015
Players are welcome to share any updated thresholds.