I disagree here.
I think it almost has 100% to do with your INTERIOR color.
I have 35% and all black interior and it looks perfect, in my opinion anyways.
One of my friends, who asked me what I had because he liked it, got 35% on his car with tan interior. He had the guy re-do it with 20% and it looks a lot better. It almost didn't even appear it was tinted because the inside was so bright.
I really think the interior color is the deciding factor. I would not go higher than 35% however.
I see what Ben is saying about the 20%, and I think that would look like trash on my car that is all black inside. Plus, it would be dark as hell. But, it looks very good IMO with the tan.
My $0.02.