My state allows 25% tints all the way around, which is what I put on my M3. I like it, mainly because it makes it hard for people to see in, but sometimes when I'm walking up to my car, it catches a certain angle and looks very black (i.e. too dark). Not a fan of that.
Getting MG M2, and thinking I should go lighter... maybe 35-50? 35 rear and 50 on the sides?