According to his newsletter, Fred Picker had trouble metering green, particularly, with his meters, including the Pentax. That's what led him to develop his own. The Zone VI modifications, AFAIK, included filtration and a new sensor that reads light more like film does.
I have an unmodified Pentax, and I don't trust the low readings. I usually meter and place at Zone VI or above and let the low values fall where they will. I still meter the low values, but it's more intuitive there.
That was with Tri-X, HC-110b and the old Zone VI graded paper and the old Oriental Seagull (it's been a while since I've done much photography.)
I'm testing the ABC, Azo, Amidol process (or will be when all the stuff gets here) so I may change my mind about the process.
I'd recommend making proper proofs, keeping records of your meter readings and paying attention. I think our analog stuff will always require intuition.
j