Windshield can show acceptable optics at 40° installation angle, whereas at 20° degrees can be completely distorted. This occurs since the light ray is required to travel longer distance in the glass as the installation angle decreases. To put it simple, the thickness of the glass that is being viewed through increases as the installation angle decreases.
The steeper the “rake’ more glass you have to look through. FMVSS 205 says 70% LT (light Transmission), ever notice that aAS1 with and arrow right below your shade band? That means 70% of the ambient light must pass through. The steeper the angles, the more glass it has to pass through. There was a time when the Federal government was going to mandate that measurement be taken from the driver’s line of sight, right now it is taken normal (perpendicular) to a point on the WS. If the government was successful in that direction, every car in the US, Canada and Europe would have to be quarantined.