hy brit
... the decision to go to top, middle, bottom, or derogate, are all based on specifics
also, in some cases, you have to consider not only the tolerance, but also the shape deviation and the control method ( for example a snake has a constant diameter, but is not coaxial )
hy peteeng
reading what you posted, i remembered talking to a guy that was used to work with a machine with 4 levels of corections, like offset, wear1, wear2, and wear 3 ... he was operating with at least 3 levels, and one level was the tolerance indeed : for example, if he should deliver +0.3..+0.5, then all corections inside last level were only between 0.3 and 0.5
it is a complex approach, definetly beyond common behaviour, that makes it easy to corelate the real dimension with the tool corection, and also to inspect periodically the cnc for calibration loss / kindly