I have 35% on both side windows. You can't tint the plastic window of the soft top, but if you have the later models with glass in the back, you could tint that the same % or even darker. Or you could even re-do the rear plastic window with a tinted plastic one sold at this web site.
http://www.emiata.com/BoxsterWindow.htm
It will match the sides better. Allot of labor involved if you decide to remove the old plastic one.
