John Parr, I have waited so long to see if you got a reply to your question that I have lost the original message, so this will have to be a bit general.
I believe it was a serious and genuine one which merited a response, so I will attempt to do so.
As an Engineer, Douglas used calculus, which he brought into his Social Credit writings at times. For example, he used differential calculus to show that banks create money.
His A+B analysis indicates that, over any fixed period, the amount of money purchasing power available to consumers always tends to be less than the total prices of the goods and services produced. With his training, it was perfectly natural for him to integrate such static information into flows. It is, of course, obvious that, if the amount of one is less at any stage, its total flow will be less. It is also obvious that production and consumption are continuing flows, not one-off events. So he was perfectly correct in doing so.
The term used is often "rate of flow", which can be confusing if it is unrerpreted as as velocity (or speed) of flow. Obviously this is not implied by the A+B statement and equally obviously the two flows are tightly associated at the same speed by the purchasing actions that link them. So "rate" in this case must be taken as the quantity of the flow, not its velocity.
I hope this helps rather than further confusing you!
John R.