In another post I mentioned a book about the Confederate Army provost guards. The book goes into great detail about a perennial -- sometimes almost crippling -- problem that plagued Confederate regiments throughout the war, i.e., straggling.

Straggling was not normally a problem associated with cowardice. In fact, "skulking" associated with cowardice was an entirely different, and much smaller, problem. Dealing with stragglers is one of the reasons why provost guards were part of an army's table of organization in the first place. It has been postulated among some historians that straggling during the 1862 Maryland campaign was so pervasive that it made serious inroads in the numbers of Southern "feet on the ground" and thus contributed to the failure of Lee's campaign.

The majority of stragglers, according to what I've read, were do-it-yourself "commissary officers" -- the unending quest for food (and sometimes whiskey, hats, shoes, and other necessaries) that the army had difficulty supplying. Then there were those stragglers who were sick and injured men who just couldn't keep up with the army, but did their best to follow the flag.

The point of all this is that, in addition to all the other factors being discussed, the number of men present on the field of battle would have been smaller because of stragglers. On the other hand, they would have been part of that "floating reserve" you mentioned.