Another way you can get a darker appearance without tinting the front windows more is to tint over the rear glass as well.
I have 20% of the front windows and 20% over the rear tint as well. You can't see into the rear of the car even standing next to it!
I also tinted the rear windows because the film used on the front didn't match up in color that well. The factory tint has a purple hue to it whereas the film used has more of a smoked greenish hue. The all now match very well with the fronts being slightly lighter.