How Is Elemental Defense Calculated?

A little while ago i saw a post explaining how since Amadel's final form has 125 defense of every element, he takes very little elemental damage. Obviously, with an ethereal, you don't do only <125 water dmg. I tested this and it manages to block meteors dealing an average of 2k water. Plz explain how reduction to elements is calculated through pure numbers of elemental defense. (ie. +100 water -100 thunder)