So with 100k hp and 60% resi you'd do: 100k / (1 - (60 / 100)) = 100k / (1 - 0.6) = 100k / 0.4 = 250k EH
You could also include other damage reductions (like armor) by multiplying them together before dividing, but I think most PvPers only look at EH from resi (since you don't know if you'll be vs a melee or caster).
