Edit: read a little closer and you have had 20% sorry....
my good friend has an h2 with 5% all around, and i had to drive his car once and it was horrible. I left the front windows down at night so I could see. It was ridiculous.
Like someone said above, headlights behind you are just a blur, they don't even catch your eye. Forget about seeing a car that's in or close to your blind spot... Or anything on a dark road without having the windows down.
20% is your best bet.