5% here...
As for backing, I just drop the windows.
One thing to consider, if you go with 35% on the doors, do 20% in the back.
Or with 20% on the doors do 5% on the back.
It will look better as when looking from the side, the tint will appear darker because you are looking through 2 windows tinted and the back you are looking through 1 tinted and 1 clear. It will look more uniform.
Just something my tinter mentioned.:thumbsup: