I would suggest 5% if you live in well lit areas at nite. Most of your driving in town, the city etc. 20% if you live out in the middle of nowhere like I do.
I had/have a saturn. Put 5% on the windows when we lived near nashville. It was great in daytime, at nite ok but on the backstreets with no lights was dark. When we moved out here it had to go. I had to roll down windows to see at intersections at nite when making turns. Not so bad in summer or warm weather, 19 degrees outside, not so great.
I did 20% on my side windows and 5% on the back glass. Still dark enough that you can't see inside, only shadows if the sunlight is the right way.