stephen...
When they (VOSA) tested mine only 47% of light got thru - well below the 70% or 75% thats allowed.
Fortunately they were only issuing tickets to folk that had under 40% that night.
When i put my car in to get tinted, i got them to do the lightest smoke possible, which i thought was 30% - it now looks like it was 50% i got fitted.