I've been pulled over before for having too dark of tint on the rear....cops i've talked to said you'll get a ticket for rear window tint that too dark. I had Zeibart do tint on three vehicles over a span of about 12yrs and they told me it was alright, but it's not.
Could be different now, but this is what i was told when i had mine done.
Also there is a certain amount of tint in the window from the factory so when a cop does a reading a 50% tint will actually show more like 45-46%. So it's actually 3-4% darker.