50% is maximum that works.
However curses reduce it. Amp by -100 and decryp by 50%.
If you get blue balled in chaos, you'll notice the Dr says 3%.
Only up to 50% works to reduce damage, but like resists you can over stack it.
DR is just a weir way of saying physical resist (which has a cap of 50 instead of 75)
So it ranges from -100 to 50 for players