Yeah xsperf, I think you have that backwards man. haha
I have 20 on my car and love it. Any darker is TOO dark and you won't be able to see at all at night... fair warning.
Limo tint is 5%.
I'd do either 20, 25, or 30.
Usually the tint shops can show you examples of different pieces of glass they have laying around so you can see it before you go through with it.