I think the darkest you can legally go on the fronts is 35%. Any darker than that, you'll get plenty of unwanted attention from PC Plod.
Back windows can be as dark as you like.
I had a 35% front and 50/55% on the rear windows on a white Clio years back and it looked nice

Most [not all!] people tend to go darker on the back because they can but stick to the legal requirements for the front. It doesn't look as odd as some people might guess having two different films.
One other thing - don't try and do it yourself. You'll never get as good a finish compared to if you have it done professionally.