from a pro
"i run a busy tint shop and get asked (and always explain) the same questions everytime, mainly, whats a " legal tint for the front side windows?"
well, to put it simply, there isnt any! the law states that the windscreen and both front side windows must allow 70% light transmission. how they got to this figure im not sure, but if you drive a modern renault, citroen, ford, vw, rover, etc, chances are that your front glass measures 65-68% straight from the showroom!
technically,the dealers broke the law when they sold the car! this is our wonderful government for you, (which is why i dont bother voting) anyway, what it means is that any film applied to front side windows will not be legal-ish.
i say legal-ish because although the police state 70%, the ministry of transport (who are the geezers with the light testing kits) should only issue an advisory if the glass reads 45% or lighter, but then if youre stopped by the feds for tints (usually outside a max power show) it doesnt matter if youve got tints above 45% or not, the law is 70%"