Here is my X3 with 15%. I will probably get the same on my '06 330i.
If you live somewhere with cops that like busting people for tint, than maybe go lighter.... but otherwise go dark.
Just taking the test drive in the un-tinted car was annoying
For guy guy in Mass who posted earlier. I have been driving this X3 (15%), a 330ci (20%) before that and a MB C230(20%) before that in MA for almost 6 years now, all with dark tint , and have never been stopped about it.