Glazing material which is intentionally made so that only a portion of a single sheet has a luminous transmittance of not less than 70 percent shall be marked at the edge of the sheet to show the limits of the area that may be used at levels requisite for driving visibility. The marks A S1 or A S2 etc., shall be used with the arrow pointing to the portion of the sheet having a luminous transmittance of not less than 70 percent and the number indicating the item with which that portion of the sheet complies.
From
here Top of the page is more in depth for the tint
70 percent is pretty light in shade, but as the code state it is actually supposed to be marked what the shade is so the officer doesnt have to guess. No marking=potential ticket. The law is pretty clear cut here