How much %pdr do you atually need?

Question:

Using Chaos Vellum as an example, with 300% PDR, having 67% Ignore Def would mean the player is hitting 1% of his regular damage on the boss.
So if a player is hitting cap damage of 50m with just 67% Ignore Def, does it means his original damage is effectively 5b per line?

it's impossible because his damage wouldnt be 1% it would be 0%.

Edit: apparently doing the math it would bring you to 1% damage but you'd also get half your damage cut by elemental resist so it would be 7.5b? (But I feel like my math isnt right so I dont want to say this is correct either.)