factory glass is generally ~97% clear...so that 3% adds into the total allowed tint. You'll notice this if you have a run in w/ a cop who has a tint scanner (I doubt that's the technical term).
Just know your local laws, and whether they are enforced or not
- If you go to a reputable shop they'll steer you in the right direction. My tint guy is very talented, and sometimes obeys the laws.