If it were me, and assuming that a 'less than zero' coating thickness is not possible, I would do a GRR on a spec range of 0.000-0.236mil and see if that closed the discussion.
Then adjust the scale to 0.230-0.236mil, how's that?
Do you have any operator measurements within 0.005 mil of the minimum?
Are you riding the line, or is this a numbers exercise when your mean coating is 0.527mil?
What is the resolution of your measuring equipment?