Hello, sorry im late with this reply but here is a website with the tint laws.
http://www.tintdude.com/laws.html
Also, yes if you get stopped by one of those cops that are really...BITCH for a lack of a better word, techincally you are breaking the law, and yes it is your fault not the tinting shop. ( They go by the rule that the civillian must always be informed and find out about the laws, like u cant shoot someone and say i didnt know that was illegal lol :P ).
But I live in FL , our state law is 28% , I put 20% and I still think thats not dark enough! Sorry but the sun here is a pain, and cops put like 2.5% in their cars then bitch about ours.. [>:]And yes the way they measure it is they put this little device that tells them the amount of light passing thru the glass with the tint, but I have been driving 5 years with 20% on and I have never had any tickets. (That one time the cop pulled me over the meter said 18% and he said thats fine). Sorry for the long response but I hope I have given you the answer you were looking for,
Enrique.