I'm not sure what the legal limit is. I got mine done almost 2 weeks ago by a local guy who has done all my vehicles and the company I work for. I have the darkest legal on the 2 door windows and I told him I wanted just a little darker than legal on the back windows, if I remember correctly it is 13% charcoal.
I have no clue what all the numbers and names for the tints are. Anyway, it doesn't seem that dark at all. At least not compared to my '99SS. The windows on it were almost pitch black (I bought it used already tinted) and extremely hard to see out of. However I never had a problem with getting pulled over for it or tickets.