Let’s use my Impale DH - Thwang - as an example.
In town, without activating anything, she has 44,378 armour which is 92.69% damage reduction.
Activating Fan of Knives / Bladed Armor increases that to 58,579 armour which is 94.36% damage reduction.
Now, let’s assume she’s running around, to get the full damage mitigation from the Endless Walk jewellery set, which is 50% mitigation.
She has an Aquila Cuirass in the cube and, assuming she has 90% hatred, that’s another 50% damage mitigation.
Her lightning resistance is 1523 which is 81.31%. This is relevant as she’s using Impale / Ricochet as her rune choice.
In GRs, I’ll regularly see her doing hits / crits in the 50 trillion range. That’s 50,000,000,000,000 damage. So what happens when that’s reflected back at the hero?
Armour mitigation
94.36% mitigation means 5.64% of the damage gets through…
50,000,000,000,000 * 0.0564 = 2,820,000,000,000
Endless Walk mitigation
50% mitigation means 50% of the damage gets through…
2,820,000,000,000 * 0.50 = 1,410,000,000,000
Aquila Cuirass mitigation
50% mitigation means 50% of the damage gets through…
1,410,000,000,000 * 0.50 = 705,000,000,000
Lightning Resistance mitigation
81.31% mitigation means 18.69% of the damage gets through…
705,000,000,000 * 0.1869 = 131,764,500,000
This is why your DH’s 1.1 million HP pool is completely irrelevant, as you’d need almost 132 billion HP to survive each reflected knife.
Skip them.