Another complexity is that the conversion of sodium chlorite to chlorine dioxide is only about 60% efficient.
MMS is a 28% by weight mixture of 80% pure sodium chlorite. That leaves us with 22.4%. From this we would expect to have a yield of 224000 PPM available chlorine dioxide. However, we never seem to be able to obtain more than 134400 PPM.
If you re-figure your stoichiometric ratio based on yield, you should find that the optimum activation actually involves adding a little more acid than is needed.
I have not been able to determine what all is involved in this, but do know that optimum activation is based on experimentation. The stoichiometric ratio serves as a starting point, then efficacy curves are developed from there. Industry is also worried about corrosion and purity, so this has resulted in using 50% citric acid in a 1:5 ratio. They are frequently using a 5% sodium chlorite concentration and this activation works the best without forming excessive impurities. citric acid can alter the taste of a food product, so they are not interested in over activation.
A lot of the information I have come up with has come directly from handbooks on water purification. You can go to Amazon and find numerous handbooks on this topic.