Originally Posted by
turbine2
I'm ex-police for what it's worth.
The 70% rule (only a max tint stopping 30% of visible light) will be for the windows in the driver and front passanger doors. The front window can stop up to 25% of visible light. Windows behind the driver, including the rear window, can stop up to 100% of light (think about transit vans that don't have visibility out the back). Halfords is correct (who'd have thought it)
The only way that you can tell is with a special meter that most traffic cars will carry. The reason for this is that different windows will block a certain amount anyway.
I've not done it myself but from what I've seen there's a knack to doing it. If you've got that knack then it's easy, if you haven't then you end up with a window that has a mottled paintwork with lines in effect from bubbles and folds and looks naff. You'll still need to get it checked to make sure the 70% visible light gets through.
Hope that helps