You are stacking it with test right? Preferably prop? A lot of people will disagree with my suggestion, but I recommend running the NPP the same dosage as the test prop. For example:
test prop: 525 mg/wk
NPP: 525 mg/wk
If you start experiencing "deca dick" lower the NPP until the problem goes away. Not everyone reacts to nandrolone the same. I know a few guys who can run it higher than test and don't have any problems. Just adjust to see how you react.