I couldnt quite understand why building profiles with the colorchecker passport was leading to reds (especially) being a little oversaturated.
I got out my (more expensive) Gretagmackbeth SG chart and suddenly all was clear.
The colours on my SG chart are much richer, specifically the colours in the center which should share common values with the passport.
Now my SG chart is a few years old and should if anything have faded, the colour should not be richer!
I can only conclude that the nice matt finish on the passport that makes it easier to light and photograph has the side effect of not being as rich in colour as it should due to light scatter.
However the profile calcualtions must be based on the colour values from the reference chart, becasue the passport colours are weaker the profile is pushing the colours up too far.
I can still work with the passport but I will need to tweak the resulting profile with the Adobe DNG profile editor, less than ideal though maybe a better path if lighting the SG chart is problematic.
(Photograph shows SG chart and passport under the same lighting, the white rectangle shows the colours that are common to the charts and used to build profiles)