JW is right. Back doors and rear window are ok, but as soon as you start messing with the front doors, they seem to hunt you down.
Our local rural cops are generally ok, but its the troopers that seem to like handing out those tickets.
I have a few friends that work in auto custom install places, and they've told me that generally speaking, anything that looks "tinted" will get you a ticket. They can't be bothered to measure it for light transmission, so, they will ticket you if it looks dark. Especially on cars with a darker interior, since cars with a lighter interior will still show contrast.
I could probably get away with a 30% tint on my LeSabre, since the interior is light grey. But on the Silverado, which is a dark grey interior, no way.
Which really stinks on days like today...we are supposed to be in the high 90s for the next 2 days.