Hi All...
I looked into this the other day. The guy tested the tints and the results were aprox 20% front v 40% rear. The tint film they had was 15, 30, 50, 70%, but was only available in carbon Black.
If you look through the glass with the door open you can see it has a green tint to it, so putting carbon black film on would mask it out and it could look odd.
I've since decided not to do it and just keep what I have.

When I had my GTI I had all the windows (side n back) tinted with 3M carbon black @50% and it looked great.

Martininwillerby