Not sure on place but here is a helpful link
Michigan Window Tint Law - TintCenter.com

This says it can be any darkness but know if u get the front windows fully tinted it will be illegal in Mi! I have looked into this and also know that most people dont have a problem with it. However, if you run into a cop that wants to be a dick (as we often see) then he can ticket you for it! Saying this i do believe the GP looks very nice when tinted and I will get mine done sooner or later