This is a difficult question to answer as there are numerous methods with no consensus on which is best. One approach uses the average bonus tolerance. Another uses the residual tolerance (bonus tolerance - position). And there are several, more complicated approaches. Each approach has its own shortcomings. However, the biggest shortcoming of all the approaches is failure to deal with violation of the normality assumption. Most position measurements are far from normal, being highly skewed, which invalidates the capability index.