I added the minimum percentage, I think it is 35%, all the way around on my 2008 Limited. It darkened the back sections up that were already smoked from the factory, but living in Texas, you are crazy if you do not have tint. The tint reflects the sun and harmful UV rays that fade and dry out the plastic and leather causing your car to age well before it should. You can noticably tell a difference in the heat reduction in the interior before and after the tint is applied. I would not apply tint over tint as it would probably be too dark and illegal, or if the base layer loosens up, you will have bubbles. If you remove the tint, use mineral spirits and a straight razor blade to remove the adhesive. You can have the installers do it, but it will cost you. Good luck!