Do you care more about privacy or solar heat rejection?
That depends on our local US State Tint law. Look the State laws up and put a film that is about 10 points lighter. For example if your state law is TX or FL of 25% VLT (Shade); buy & install a 35%. 35% car tint plus window will net 25%-29%. Car windows seem to be getting darker and darker each year.
Check out your state laws directly buy conducting a search with your state, “CA state tint law” for example to get the latest update on any changes in your state. IWFA has a good state tint law site too. http://www.iwfa.com/Portals/0/PDFDocs/Law_Charts/State%20Law%20chart%2005.22.17.pdf
As per Texas, the dps.texas gov site had the following about windshields:
Sunscreening devices can be applied to the windshield if all of the conditions below are met.
Sunscreening devices must be applied above the AS-1 line. If there is no AS-1 line, sunscreening devices must end five inches below the top of the windshield.
Sunscreening devices may not be red, amber, or blue in color.
Sunscreening devices, when measured in combination with the original glass, must have a light transmittance value of 25% or more.
Sunscreening devices, when measured in combination with the original glass, must have a luminous reflectance value of 25% or less.
A clear (un-tinted) UV film is allowed anywhere on the front windshield without a medical exemption being required.