How could someone not know this. This is childs math. Weird OP struggles with this

i thought about it again. i think it goes like this:
for any armor with innate %ed you take the highest base def possible +1. for hellforge plate base def is 421-530, thus 531. this you multiply with the %ed, in this case 100%+15%+200%=*3.15 (don't forget that it is +%ed so you have to add 1 to the multiplier). add the +15 from el rune. and then round it down.

531*3.15+15=1687.65=1687 final def as calculator says