非正态分布，PPK计算？

ISO method:

Pp = （USL - LSL）/（X0.99865- X0.00135）
where:

USL = Upper specification limit

LSL = Lower specification limit

X0.99865 = The 99.865th percentile for the specified distribution

X0.00135 = The 0.135th percentile for the specified distribution

Minitab method:

Pp = (ZUSL - ZLSL)/6s


where:

Zlsl = F-1(p1)

Zusl = F-1(p2)

F-1(p) = p * 100th percentile of a standard normal distribution

p1 = Prob (X < LSL)

p2 = Prob (X < USL)

X follows the distribution used in the analysis

X0.99865和X0.00135怎么算呀？