IMO don't mix %'s (front and rear) - doesn't flow well.
I have 20% but wish I had 15% like my A4 did.
I've had 35%, 15%, and now 20% on 3 different cars (all black int. 2 black ext.)
35 was way too light.
20 is good for me
I think 15% is the way to go for you. Its dark enough where you need to lower your windows to talk to people at night, but light enough to see clearly.
Be SURE to ask what brand film they are using.