The Ultimate Effective HP Formula

First off; a little background. This 10 armor = 1 all resistance thing is thrown around too loosely, there is a very crucial caveat to that rule. When Armor is very close to 10 times your reistances, 10 armor = 1 all resist. It has a domain because your effective HP is equal to:

HP*(1/1-%reduction due to armor)*(1/1-%reduction due to resistances)

To exhaustively calculate EHP coefficient from armor and resistances, use this equation:

1+A/50V+R/5V+A*R/250V^2

For A=armor, R=resistances (i use my average resistance for simplicity), V=arbitrary constant.

Technically V is the monster level of interest but it doesn't make a difference for comparing item sets. For reference, A1i mobs are level 60/61, A2i=62, A3i/A4i=63. Further, keep in mind that fire resistance is far more valuable than other resistance types when comparing single resist items (mortar and fire chains are the most damaging, hardest to avoid special affixes in the game for ranged and melee respectively).