Anything 20% and down you're asking for trouble. I have 35% and still got pulled over...... granted, he only hassled me because he didn't have time to clock me when he pulled into the service entrance and I was driving
WAAAAYYYYY too fast...... So I got that and a warning for an "overly obtrusive plate bracket" ... yeah BS I know, but he certainly needed an excuse to humiliate me..... Another good thing to check is what a second offense will cost you. I fought mine and got out of it ($174) but had I not, and got another since (that was the only time in 4 years tinted) a second offense in my state is
$900
..... going lower the hassle increases, I had a buddy that had 5% in his prelude and the state trooper made him peel/scrape it off on the spot.