This is the little bugger from db\misc.txt that is harassing you:
Code:
# Supply Usage Land Battle (value below * normal supply consumtion)
4.0
Basically troops in combat will have four times higher supply consumption. This WAD and has not been changed in my mod. Only thing I changed in this regard was to lower artillery bombardment supply cost from x6 down to x4.
However, the calculations for supply demand are usually too high when your troops are advancing, because the game believes the x4 demand is applied 24/7, so even if your troops fight only 2h on a day, the game will believe that it would need four times the regular supply for the whole 24 hours.
Or in short: You don't have to produce as much as the game asks for, just assign 25-50% more than your regular peace time supply consumption and it should work fine. Check your troops supply storage from to time to see if the assigned amount of IC is enough.