Realistically it's all pretty low. Would they need more water? For sure. But 1.5L would likely keep them alive, especially if they were campaigning during the cooler, non growing seasons which is the only way you'd ever get your numbers to even approach 100 or 300 thousand as you would be relying on your farmers as the bulk of those troops.

The bar I set is not 'well fed'. 2500 calories is what I would consider to be survival level given the physical expectations of an army on the march and in battle. For reference, a modern American 24 hour pack of MRE's contains between 3500 and 4000 calories. Likewise, water I estimated at a low end, since it is quite likely that an army on the move would almost certainly follow a river, where they could access flowing water that could be used for cooking and even be somewhat 'purified' for drinking through the addition of alcohol or vinegar.