Last MOT our Opacity measurement was 0.1 iirc (can't remember the units), he double checked to make sure the probe was in the exhaust!.
All standard, one of the cleanest cars he'd had on the exhaust tester apparently.
My tester had a similar issue, from memory he got 0.22 the first year and didn't believe it. He has had to become used to my engines low figures over the years I gave it a full and very thorough service, just after buying it, as I always do when buying a new to me vehicle.