I would recommend everything but the 18%
That was way too dark. I think at one stage I almost ran someone over as I was turning right, because I couldnt see to the right of me.
So 18% is way too dark, unless you are prepared to drive with your windows down the whole time.
28% isnt too bad. Not as dark as the 18% but probably as dark as I would want to get.
I think most people will be happy with either the 38% or the 28%. These two seem to be the most popular, according to Robert
I'll take some photos tonight (if I dont forget), but its hard to tell the difference because it depends entirely on lighting conditions