Such a calculation would be very hard to do as it involves 5 variables which includes the pH of the ammonium thiosulfate 60% solution which is variable from batch to batch. But the amount of citric acid carryover should be small and of little consequence. Just let the prints drain well before putting them in the fixer.Gerald, would you be able to estimate how many ml of citric acid stop would it take to bring 1l of a mix of (ammonium thiosulfate 120g + sodium sulfite 15g + sodium metabisulfite 5g) below pH 7, say to 6.5?
If you are concerned about the pH of the fixer it is easy to check it periodically with some pH paper of the appropriate range. If it becomes too acidic then just add a small amout of base.