Just had my 08 done a little while ago. I went with 35% on the fronts, 20% looked too dark when compared with the factory privacy glass. The installer where mine was done had sample "squares" of tint that they quickly put on to aide in the decision, see if your installer can do the same.
FYI, I had 50% put over the privacy glass and it looks great! The factory glass is too light IMHO.