I have an appointment this Tuesday for xpel ceramic tint, and I want to go as dark as possible on the sunroof. What is the darkest I can go safely for the roof? Shop says 20%, and I have no reason to doubt them, but I wanted to see if anyone has gone darker successfully. They say it is due to the expansion of the glass, too much and it could break, that would be a hell of a surprise rolling down the freeway!