Here are the laws by state.
State Window Tinting Laws
As an example, (Indiana) it's 30% for the sides and 30% for the rear.
This means that of the 100% of light that passes through a non tinted window, only 70% passes through that particular darkness of tint. 30% of the light is blocked.
I REALLY enjoy driving my car because of the tint during the day. When I drive the Cavalier (for work) it's not tinted and I find myself squinting a lot (no tint + a viser thats too short really irritates me)
Edit: Here's a link that explains some other things. Tint Laws
I don't think that is right. The darkest tint you can get is like 5%(limo), so that would mean(by your example) 5% of the light is blocked... not correct, 95% of the light is blocked, only 5% is let through. Know what I mean?