smellie,
As far as subtracting the number of rifles from the number of men brought from France, I can account for the discrepancy by at least two units and show why this method of accounting is probably not too accurate.
Two of my uncles were evacuated from Dunkirk...One a Captain in the Royal Engineers and the other a bandsman. As the first would have been armed with a revolver, while the other, being assigned to ambulance and stretcher bearer duties, thus rather busy at the time, probably wouldn't have been armed with a rifle either. Obviously there were many other service personnel not issued rifles, so to assume each man getting back without one represents one rifle left on the beach, seems a bit of a stretch.
Don't remember at the moment where I read it, so will have to do a bit of research, but, I do remember at least one account of the evacuation stating that most men did bring their small arms back with them.
Terry