Many software including MINITAB obtain the ppk no matter if "is normal" or not.
The first step to compare results, check if MINITAB obtain the same value without any transformation (I am pretty sure that the results will be equal), and if not is equal, try first to see if changing options in the configuration screens make them equal (A friend of my obtained two different values in 2 differents reports of MINITAB and checking the options changed both to the same value).
use a lower spec of 0 if is not the case, the indicators like ppk take in account that the target is centered on the specs.
Agree that a transformation (and I really like Box Cox) could transform your distribution an the value obtained will represent with more reality the data but as I think "If you are going to jump into a pool is better to try to see the bottom of the pool before jumping than not doing it because you have no rule to measure it."
. An Indicator by it self is better than none.
note: if you are going to transform the data, you will have to transform the specs also.