Effective Hit Points are the hypothetical amount of health you have when you have reduced damage buffs.
To determine your effective hit points, divide your total health by the percent damage you take from attacks. For example, Iron 4-Skin gives you an effective hit point total of 117.65 (100 ÷ 85% = 117.65). This is 85% because you are taking 15% less damage.
Note: Bleed damage is not reduced by these buffs.