5% always. It's illegal in FL but I've had it on 2 different cars totalling 10 years and have never had a single peep of anything from police, even while pulled over for other things like speeding. Maybe its the sun we have here, or the car is clean otherwise, not sure. It's rare here for people to have issues although I have heard of it... something like $95 but if you remove it and bring it in to field office it gets lowered.
To me its worth the nice looks, protected interior and privacy on the road.
Edit: If the cop uses the device to measure light through the window you can attempt fighting it over lack of calibration of the device. How do you and/or the cop know that its accurate?
"Fast she goes...in the city of Overdose"