Question on Extraction on Boss Mob

Fiel doesnt extrac the pdr either how do they know the pdr too?

PDR is not too hard to estimate. just attack a mob with 0% PDR and 0% boss and see how much your damage is reduced. If you normally hit 100k and you hit 70k, the mob has 30% PDR, if you hit 50k it has 50% PDR, etc.