When I calculate the wall thickness, I take the smallest POSSIBLE thickness. If the published thickness was 0.25" and mill tolerance was 12.5%, I would use (1-.125)*.25"=0.21875" as the smallest thickness.
If I was calculating the thickness of the pipe based on 31.1, 31.3, 31.8, etc. , I would make sure I chose a pipe (don't forget to subtract mill tolerances, thread allowances, if needed and corrosion allowances) whose thickness was greater than the calculated thickess above.
am I confusing?