How much %pdr do you atually need?

Empress and Chaos Von Bon have 100% PDR, Hard Magnus and Chaos Crimson Queen have 120% PDR, Chaos Pink Bean and his statues have 180%, Chaos Vellum and Lotus have 300% PDR, and basically every other boss has less than 100%. How much Ignore DEF you need depends on what bosses you want to do.

You need at least 67% Ignore DEF to do more than 1 damage on the 300% PDR bosses (because that's the minimum it takes to reduce their PDR to 99%, meaning you would do 1% of your damage).

At ~67%, 100% PDR bosses have 33% PDR (you do 67% of your damage), 120% PDR bosses have ~40% PDR (you do 60% of your damage), 180% PDR bosses have ~60% PDR (you do 40% of your damage), and like I said above, 300% PDR bosses have ~99% (you do 1% of your damage).

I've seen people recommend you have 93% Ignore DEF, which means you would do 93% of your damage on 100% PDR bosses, ~92% on 120% PDR bosses, ~87% on 180% PDR bosses, and ~80% on 300% PDR bosses. If you're fine doing less damage, you can have less than 93%, and if you don't plan on doing Chaos Vellum or Lotus, then I wouldn't worry that much about it getting to 93%.

And no, Ignore DEF has no effect on elemental resistance.