i understand what you're saying but you don't need a meter to tell what darker tints looks like if you know what 70s look like.... 70s just have the slightest tint of shading.. you can barely tell it's tinted when it's 70% so it's obvious when it's darker.

any dumbass cop will be able to tell if it's illegal tints on the front window because any obvious tinting is < 70% light transmittance.