Elemental Weakness from Red maps is Lv15 (-34%). So after beating act10 eventually aim for 109% in brackets.
Monsters can then still apply Frostbite/Conductivity/Flammability which will further reduce your resistance to the specific element. I do not see any specific level noted in the wiki, but expect it to be another -30%~-40%.
A curse removal flask is tops, I would always recommend one.
SoulTax : They set it so the -60% is now removed from the brackets on the defense tab. So you do not have to hit 135% / 169% in brackets. You still have to get those numbers, but the brackets are now also reduced once you go past act5/act10 so after act10 if you see 75% in your defense tab you are sorted.
I know. That was my point. That they always recommended 169% when the debuff was not global. Which equals 109% now that the debuff is global. But I see that it is because of the curse immune flask to prevent any further reductions.