im fairly positive that in iowa which is the state is the op is in, you can have as dark as you want on the back window. i knew a guy with black vinyl on his back window.
Law in Iowa states you can have 70% on the front 2 windows (driver and passenger) and no limit to rear vehicle windows. I had 20% on my subaru and constantly got popped for it, though I only got warnings to take it off and i told the state patrol officers no every time. I installed it myself knowing it was illegal but I have an issue with the sun giving me headaches while driving and I wear sunglasses with no avail.
There is a stand that I have seen used when going to court for this , although it was in Illinois BEFORE they dropped their tint laws, but a guy that was part of an area subaru club got popped for his tint on his wrx and got a $350 ticket for it. He went to court and his stand was it is a safety standard when having tint on the windows since the window will not shatter in the case of an accident. And he WON!
It varies state to state and most states have dropped the tint law because of the old satute of police safety due to the fact that the cop cant see if theres a weapon being wielded even if theres no tint on the windshield. Another reason for the drop is that cops were spending to much time worrying about minor traffic stops such as tint and letting more serious offenses go by.