on this kind of stuff that double dips its usually full value.
as its based off of damage thats already cut, if it were cut in half again, thats the double dip. they have corrected when that has happened in the past (jabs, etc.)
regardless its going to be OP
you already get better healing from stacking damage in this game, and this is an item that does not even try to hide it. annoying stuff is going to get worse, tanks sitting in zergs, un killable 1v1's.
this is probably the worst and most toxic mythic added to the game, really hope it does not go live.