Michigan law doesn't state anything about the exact darkness you can have your windows tinted but Michigan prohibits applications of a solar reflectivity of more than 35 percent.
Also in Michigan you're only allowed to have the top 4 inches of the Windshield and the front side passenger and drivers side windows to be tinted.
If your car has back doors and a back seat you cannot have those windows on the back doors tinted and you cannot have your rear window on your vehicle tinted according to Michigan State Law.