A few comments. Some of the issues were already tossed back n forth between me and Richard on that linked old thread. But other than those, it would have been nice to compare the unmodified Pentax spotmeter values to the modified meter ones. Meters can change over time; and unless re-calibrated, all those ZV I modified ones are pretty old by now, and I don't know how stable the internal filters are either over time.
The MacBeath Color Checker Chart is indeed a superb help for such instances if itself kept in fresh unfaded condition. But one has to understand how these Charts work, and how different colors saturate at different density values. Light meters are not color densitometers. And color film is not the human eye. Nor do these printed color patches on the Chart necessarily behave in reflectance like similar-looking colors in nature.
I've had some Hollywood cameramen tell me they love the modified meter for shooting color neg film, others that they prefer the unmodified version. But there is obviously quite a range of color films just like even different kinds of panchromatic black and white films. There are just so many variables that one really has to be "married" to a specific meter awhile to understand its real personality. I found the unmodified Pentax to read identically to the Minolta Spotmeter F, but prefer the more intuitive simple dial method of the Pentax. But thank you for your own contribution.