My Mazda6 had 30% llumar tint on every window, except the windshield. I wouldn't recommend getting anything darker than 25 or 30% VLT, as it will look ghetto and impede your night driving.
I'm probably not having the Fiesta tinted. The setup I had on my Mazda was technically illegal in NJ and I got about 6 tickets (mostly from the same cop) and went to court 3 times over it. Cops will do things like write you one ticket per each illegally tinted window and make them mandatory court appearances. Pretty weak.
I plan on not running a front plate on the Fiesta, so that'll be my one illegal thing.
If I lived in an area where tinting the front windows is allowed, I'd probably get 40% film.