i tried reading the cali vehicle code crap, but i hate reading legal garbage. but from what i understand, as long as you have side mirrors, you can paint your rear side and rear windows with black paint if you want. but as for the front side windows, 88% is the maximum tint allowable.

http://www.dmv.ca.gov/pubs/vctop/d12/vc26708.htm

personally, i have 50% in the front and 35% in the rear. my friend has 50% front and 5% rear. we've had it for years with no problems. another guy had 35% in the front and got pulled over for it after about 2 months. he re-tinted with 50% in the front and no problem.