Looks like in Boston, 35% is allowed :
For most vehicles, aftermarket tinting that registers at least 35% on a tint meter on side windows immediately adjacent to the driver and front passenger seats, and side windows immediately to the rear of the operator and passenger, is acceptable. The rear window may be tinted to the 35% level, but the vehicle must be equipped with two outside rear view mirrors. The windshield may only be tinted down to the AS-1 line, which is usually located in the uppermost (6) inches of the windshield.
http://www.mass.gov/rmv/faq/inspection.htm#13
Nick