well, when you ask how dark you should get you have to think of a couple things, such as do you not want to be able to see anything unless the light is on, or do you just want it a little dark.
I believe that they go by percentages... obviously, you can think of 0% as no tint, and 100% as not at all transparent. I would guess that limosines with blacked out windows have somewhere around 80% tint and that the average car's factory tint job is somewhere around 20-30%
Personally, I'd go for around 40%, I would guess that you could see some things inside if you looked hard enough, but with a light behind it it would be clear.