I'm not sure where alot of this "30%" rule is coming from, but I think it's super important to set it straight.
In California, the window tint rule is
Vehicle Code 26708. In short, you cannot tint the Front windscreen, or the front-driver's-side windows with any film that is not Transparent. You can put /clear/ uv coatings on the window, but technically any tint whatsoever is illegal. Also, if you have any tinting on the rear window, then you must have mirrors on both sides of the car.
I call window-tint tickets "excuse tickets" in that the cop will use it as an excuse to pull you over if he thinks maybe you can be busted for something else. That's my theory to explain why you see a ton of cars with tinted fronts that never seem to get a ticket, and why I never got a ticket in the Infiniti with 20% on the front windows, but why in my little lowered Yaris S or Scion xB (original), I got pulled over for it constantly. They see your car and bet you're doing something wrong, and use the tint as an excuse.
If you ever get the fix-it ticket, remove the tint and take the car to a police station in the same county where you got the ticket. They'll sign off on it and it's like $15 to pay at the courthouse. There are a few window tint places, including one in my city, that will remove the tint for free, and will re-tint after you get the ticket cleared. Not sure why you'd want to, but they are out there.