From what I've read, NJ allows any tint on the rear sides and back window, but nothing on the front sides and windshield.
So with that in mind, know that you will get more looks from the police the darker you go. Also, tint darker than 35% will have you putting down the front windows and using the side view mirrors to back up. Finally, insurance companies can deny coverage if they find you have illegal tint and are in an accident and are at fault.
State Window Tinting Laws