Quality Digest

I've read that when determining process capability for a non-normal distribution, the process spread can be evaluated by finding the 0.135 and 99.865 percentile points using a probability plot. From this, I infer that the sigma value to be used in the capability calculations would be 1/6 of the spread. Using Minitab 14, I generated a column of data and then used the Individual Distribution Identification macro to obtain the percentile points. That macro concurrently reports the descriptive stats. (The generated data was Weibull 3-parameter, and the Identification macro was set to evaluate for this distribution). In this case, the std dev was reported as 2.25, while 1/6 of the "process spread" was 2.77. Moreover, the Minitab non-normal capability analysis (again selecting 3-parameter Weibull) reports only Ppk (1.22) due to 1-sided tolerance setup; Cpk is absent. Now, when the data is evaluated using a Box-Cox transform, Ppk drops to 1.00, and a Cpk value is given. So, my question is: Which is the better way to calculate process capability for non-normal data? It would appear that, in this case, the quantile method would yield a more optimistic result.