if you have a dark interior everything will come off looking a little darker.
i have 35% over the factory rear windows, 15% on the front two, nothing on the windshield. it looks even and you can only see a silouette through them even on a sunny day.
also, shut your sunroof cover to block out more light.
don't do the windshield.
expect to pay a minimum 250 for all the windows, more for the windshield