from experience with the local constabulary on this subject , been bollocked for this before , the rear windows , back and sides , you can pretty much make them as dark as you like as it just makes your car like a van and they dont need back windows . but the windscreen cannot be tinted at all and the front side windows must be 70% light transparent or you can be made to have them removed at the side of the road .
but they can only make you remove them if they can prove using a light meter that the windows are breaking the law , and very few coppers carry one of these around with them . as with speed cameras this device has to be tested regularily .
hope this helps a bit.
p.s. just a note to say , if you use your car a lot at night tinting can be a major pain as you cant see nowt through it especially reversing.