diminishing returns starts when you get past 50% mitigation from armor or prot alone.
crazy mathematics gave me this closest approximation so far :
- keep in mind that you have 2 "sources" of mitigation : immunity to damages (XX% value), and armor for physical / protection for magic (integer value)
if armor>50*D then Mitig = (100-(armor/D))*Immunity + (100- (5000*D)/(50*D + armor))
if armor<50*D then Mitig = (100-(armor/D))*immunity + armor/D
Try it with D=219.6 for armor and 73.7 for prot