Use bare-bones divisions - straight rifles, but with the same number of battalions as your front-line units. That means if you have line artillery use a rifle battalion to re[resent them as well as for each support unit. Then once trained you'll have most of the personnel so you won't lose experience due to fresh bodies coming in, just unfamiliar equipment.
But don't do that with your line units. The AI can surprise you even on historical, especially once WT gets to 100 and enough non-historical stuff has happened.