You use the same calculations as equal bilateral tolerances. The calculation uses the tolerance spread and the process spread, Cp=(USL-LSL)/6sig, and is not concerned with the "target" value. Even Cpk uses the
average, not the
target: Cpk= the lesser of (USL-Xbar)/3sig , (Xbar-LSL)/3sig. Even though the tolerance appears to tell you to run to a target value below the "middle" of the specification, capability treats the tolerance spread as the
entire dimensional opportunity for your process to run to, and your ability to keep your process within that entire dimensional opportunity.
One might argue, though, that if the use of an unequal bilateral tolerances is a
desire to skew the location of the data to one side, that reporting Cp=(USL-LSL)/6sig alone would be sufficient to prove you are capable, because Cpk values will appear much worse if you do do not run to the center of the specification. The whole
point of the Cpk calculation is to see if you are centered! Good luck!
