1. yes the percentage refers to the amount of light that gets through
2. if you don't want your car to look like a limo, stay away from 20% and 5% - these 2 shades are considered limo tint. That leaves you with 3 choices
35% (my recommendation),
50% (O.K. but not dark enough for my taste), and
75% (looks like you don't have any tint, it's basically put on to filter out the UVL)
3. I would stick to the same shade all around.
NOTE: Double check with your police dept to see what the Law in your state is regarding tint. ie. in California any shade is o.k. as long as you don't tint the 2 front windows and the windshield. In Hawaii, you can tint all your windows 35% or lighter. (except your windshield)
1986 Gold 300E 4Dr automatic