The planning factor we used for organic logistics in the mech world was
Mech/Tank company = 1 fight, after that you need ammo, but maybe not fuel (depends on offense / defense / mobile defense)
Light infantry company = 1 fight in the offense, 2 in defense since you can pre-stock, and light infantry doesn't really fight a 'mobile' defense
Mech/Tank battalion = 8 hours of operations w/ organic assets carrying their basic loads
Light infantry battalion = 24 hours of static operations; mobile ops for light guys are entirely dependent on the assets you add to them to help move them around, so no rule of thumb planning factor
Heavy brigade = 24 hours of operations w/ organic assets carrying their basic loads
Light brigade = same as battalion, but 48 hours of static operations
Artillery brigade = 24 hours of operations w/ organic assets carrying their basic loads; if the battalions are parceled out, then it depends on the support they get from their maneuver brigades
Once you get past 24 hours, you're depending on divisional trans assets and whatever corps-level reinforcements they offer, but that goes back to what James was saying above that at the division level you're into some forecasting voodoo.
Now, things that are never planned in wargames
-- CASEVAC / personnel replacements
-- backhaul of KIAs (leave no soldier behind)
-- WSRO operations to bring damaged equipment back into service
-- 'clean' vs 'dirty' routes in the event of a chemical attack / decon locations
-- FOOD! Guys gotta eat at some point
the other big issue is that all of your support assets - fuel / ammo depots, maintenance collection & repair locations, decor sites, field hospitals, etc - take up space. That's a big deal. You're assuming a lot of room to maneuver around behind your main lines that doesn't necessarily exist once you put all these assets on the map somewhere. That's particularly important if you have a robust artillery presence in the game. Those artillery assets are moving around a lot of the same real estate as all those things listed above, and none of them want to be anywhere near the artillery, b/c those guys always attract a lot of counter-battery fire that no one wants to be anywhere near.