The % that is referred to is the amount of light let in i.e 35% So if the % is less, the tint is darker as it lets less light in. I have 20% on my back window.
I believe the darkest legal tint refers is so that visability out of the car is still good rather than looking into the car. With my tint, it is quite black from the outside, looking in but looking out when driving, there is barely a difference IMO.
When I got to the tint place, the guy already had a package he suggested which was the 35% windows and 20% rear which I think is being offered to all people. The 20% rear normally costs a bit more but they are doing it inclusive from what he said to me.
HTH and if anyone disagrees, please correct me