I've just had the rear windows in my Passat tinted. I want it to be the same as the original privicy glass, so we tested the rear windows on my Mk6 Golf. They were 75% i.e. let 75% of the light through. We also tested a new GTD. This also measured the same, but looked darker. Most probably down to the black headlining.
As for tinting front windows, nothing other than the factory green tint is legal. Some people get away with a light tint if it doesn't look too obvious, but are taking the risk.