Ron,
I think the only thought that would have been firm in Wellington's mind would be that any retreat would not be towards Ostend. I think we see the Allied strategy much alike. I do think that there were substantial numbers of men flowing through the French depots as both drafts and units. I don't have my Houssaye to hand, but I seem to recall the number of drafts en route was substantial - the number 15,000 comes to mind, with tens of thousands more in process. Regnault also indicates that 8 complete battalions were en route to the north to join their regiments. Considering the number of personnel reporting into the depots, and the buildup in the NG, Napoleon was assembling a sizeable force. That said, even if he'd had the time to build up a field army of 400,000 men it would still be long odds against the 700,000 or so committed by the Allies plus the additional corps en route from Russia, plus the additional corps being formed by Prussia. I figure Napoleon must have calculated that he was facing 2:1 odds now or later. However, he stood a better chance with those odds using a smaller army against smaller opponents instead of waiting for another Liepzig. That is, he could be more nimble in Belgium in June with 120,000 men, than in Champagne in August with 250,000.
David