I thought Kodak stopped producing Double-X decades ago. They replaced it with Tri-X. Where did you find Double-X?
Also, have you tried reducing the pH some? 8.6 is high for a PC developer.
What does Sodium Citrate do differently to image-quality compared to other alkalies such as Borax or metaborate? It's seldom used in developers, so I'm curious.
As an experiment, I suggest reducing Phenidone to 0.1 g (instead of 0.15), because that will make ratios closer to what I've found gives finest grain.
Anyway, thanks for sharing this. It's an interesting PC developer.

Mark Overton