I either read it on a ticket or a cop told me that having ANYTHING on your headlights or taillights is illegal (at least in California). Not sure as there are many laws pertaining to the lights on a vehicle.
I had the top portion of my headlights tinted only covering the turn signal and parking light. I added turn signals to help other cars (although stocks were still visible) and still got numerous tickets for the tint.
30% wouldn't be dark enough to warrant a ticket I wouldn't think, my lights were just as black as my car
