I'd try your state's government office...
Well, I did a little digging, and this is getting interesting...
PA state law does not list a specific percentage, but rather says, "No person shall drive any motor vehicle with any sun screening device or other material which does not permit a person to see or view the inside of the vehicle through the windshield, side wing or side window of the vehicle."
Yet, the attorney general claims, "light transmittance through all vehicle windows must be greater than 70%. [...] Some window tint retailers try to defraud consumers by offering illegal tint levels of 50, 20, or even 10% light transmittance levels."
The attorney general also claims, "The Federal Motor Vehicle Safety Standards prohibit tinting of passenger vehicle windshields and front windows where light transmittance is reduced below 70%, just as in Pennsylvania."
However, the applicable standard, FMVSS 205, does not state a percentage, and other states' laws comply with FMVSS 205 with percentages ranging from 25% to 35%! I guess I'll just have to try something in that 25 - 35 range and hope for the best.