Adding a sodium sulfite/wash aid to this single-fixing bath is simply not going to increase the capacity by 4x. From what we've been discussing, it may (emphasis on the "may") increase the fixer capacity a small amount, maybe even 10%, but likely less.
You say it doesn't, yet Ilford claims it does ... and I would assume that Ilford did some serious testing before they published their numbers. Did you?
Note that Ilford would love to sell us more fixer concentrate ...
Trying to be the best of whatever I am, even if what I am is no good.