The percentages are wrong, the lower percentage the darker the tint.
A 50% tint will let in 50% of the light, a 20% tint will only let in 20% of the light.
http://www.dynamictinting.co.uk/the_law.html
I had mine done, 50% all round on my last E89.
If i get my new one done it will be 50% light smoke on the sides and 35% smoke on the rear as I think the rear screen was a bit too light last time.
I wouldn't go darker on the sides as it stands out too much, and if the old bill pull you over they could make you remove it there and then (if they are having a bad day!).
I think the E89 comes with a 70% tint as standard which is the legal limit, so there wasn't a huge difference, which is what I wanted. Didn't want to look like a drug dealer!!
Whatever you decide it's still illegal :wink: