You must have at least 75% light transmitting through the driver and passenger door window.
All windows behind the B pillar, can be as dark as you like.
When i first purchased my Civic, i had a "light smoke" tint applied to the front door windows, and a "medium" tint applied to everything behind the B pillar.
It was just dark enough to reduce glare and heat, but without looking chavy, lol.