My understanding is that thwres a a federal limit of like 30% light being blocked for windshields, with most states having stronger regulations.

My state, Mass, only allows any tint in the top 6" for instance. BUT, its really easy to get a waiver, overriding that. All it requires is taking a doctor's note documenting photosensitivity to the DMV.

Back in highschool, my friend's mom had glaucoma, and got her car tinted. Because he was on his parent's insurance, that made her a registered user of his car, and also entitled him to tint his out. He had his windshield tinted to about the darkness of a medium pair of sunglasses.