Not sure what states you guys are from, but here in boston, if you have 20's and unlucky enough to cross a cop who notices, your pulled over, paying 250$ and ripping your tints off, if the statie doesnt do it first.
Our limits 35, which SUCKS.

 

15% would look perfect for your black car. Especially because you have black interior, the tint will look a bit darker than it really is. I noticed you are located in SoCal. Did you tint your car yet? If not, shoot me a PM.