Gotcha.
I've been really lazy with tinting any of my last 5 or 6 cars. Back in the day I had a 1994 Toyota 4Runner and had seriously dark limo tint on all windows (including back window, which came down on those 4Runners - my favorite feature of the car) except the windshield. I don't recall the % but you could not see inside the car from the outside, even with your nose up to the window.
Anyway at night I'd often open the rear window so I can use the rearview mirror and actually see something... so I thought immediately of that when I read "tint the windshield"
I have been meaning to get a tint for the X6M though (darkest legal I can go) and will see about tinting the windshield. It is an interesting suggestion.