How much %pdr do you atually need?

First you need to calculate how much PDR you need to do any damage to it, so in your 120% example you need to ignore 20%. 20% (amount you want to ignore)/120%(total PDR) gives you 16.67%. After that you multiply 100-that percentage (so 100-16.67 = 0.833) by the defense the monster has to see how much damage it resists. In this case it would ignore 0.833 * 120 = 99.96% damage so you'd do 0.04%. After that instead of 16.67% ignore you'd use 18% or whatever amount of defense ignore you want instead. Then you can start changing the 120% to whatever defense the monster has and start from the beginning if you want to change monsters.

The core concept is that of diminishing marginal returns - the more defense ignore you have, the less an additional point of defense ignore will increase your damage by, as Grey said.