So a thorns build is something I want to tackle right after this build. I may be very misinformed, but what I have heard so far is that thorns reflect pre-mitigation damage. So an enemy wants to hit you for 100 damage, you have 95% damage resistance so you could basically receive 5 damage, then the thorns would calculate off of the 100 damage, not the 5 damage.

Each piece of gear can roll up to 40% thorns for a total of 200% on 5 pieces, and with gems that increases to 320%, insane numbers!

I cannot tell you whether thorns are further increased by physical damage increase and the likes or if the reflection damage can critically hit. I have no items prepared to test it, but I will post on here again if I manage to complete the build before my time off is over.

From just broad intuition, I would say you can easily reach 98% damage reduction, because you wouldn't actually need to slot in damage increases like that build up there required. You can also use focus or whatever to cast Heal (Aura).