You can have as much tint as you want behind the B Pillar. Between the B pillar and A pillar - allowed up to a 30 % tint. That means on the drivers and passenger doors the tint must allow 70% of light through. Police do have devices in their traffic cars to test. But any decent tinting company will also be able to advise.The windscreen has to allow 75% of light to pass through.
So do my plastic door windows that are now a nice deep brown count as an excessive tint?