Thank you, Gerald. I will keep notes on the evolving pH of the fixer as I work with it, what an obviously simple suggestions, many thanks, indeed. I had no idea that ammonium thiosulfate 60% solution comes at different pH, batch-to-batch. I have assumed it to be more-or-less constant. I suppose this means even Ryuji's recipe needs to be adjusted to get it into pH 7 upon making, I suppose by varying the amount of sodium metabisulfite.
I also want to thank Rudi for his PM suggestions re ionic strength.
Still, why do I have a feeling I am rediscovering something that must have been done by quite a few others?