What is the theoretical basis for calculating the net gain of ATP...

There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another. The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation. None of the intermediates in the pathway are utilized to synthesize any other compound. Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages