the legal limit in NC is 35% on the front 2 and 0% on the rears.
i had 5% on my front 2 installed and 5% over my stock 20% installed on the rears, cost me $60 becausei go there so much and their tint is the best tint i have ever seen.
You will quickly realize that some companies use tint that isnt very good and you can tell because they will tint your windows at 5% and you can see very easily in, this is thin, cheap tint.
normally here in NC the best tint is the tint at the beach, they get the darkest there, i also heard sunny states like FL and TX also have some of the darkest tint. So ask to see a sample IN PERSON, not a picture and it should look as follows(for good tint):
35% - you can see through fine but when in a shady spot the tint will be noticable.
20% - you will be able to see a sillouette and facial features in the sunlight.
15% - you can see a sillouette but no facial features in the sunlight.
5% - nothing, this is blacked out unless you dont have a banner on the front and the sun is setting/rising and shining in your front windshield, then you can somewhat see through the side front windows.
hope this helps!!