?? Tinting My Scion TC ??

I have an 08' Scion TC. Tinting the windows will be the first "mod" i will do to the car but I don't know how dark the windows should be. I want them dark but i dont want to be getting pulled over for them being too dark. I've been thinking about 20%, for anyone who has this percentage tint. Have you run into any problems with the police about the tint?