One thing many people don't realise is that standard tinted windows in cars and vans are often already close to 30%, so if you then buy 30% film and stick it on top you're well over 50% and driving illegally. It's one of the things loss adjusters check after an accident, takes 5 secs with a little portable meter (we use them at work), I think some police officers have them as well. As mentioned, this only applies to the front side windows and windscreen, the others can be as dark as you like, you can even paint over them if that's your bag.
Edit: Actually I'll bring one home next week and check my standard front windows and post it so we have a T6 baseline