Make the production log warn in WU terms, when Warehouse is going to overload.
Today, we are warned when production will stop for lack of raw materials.
Product - Consumption - Overproduction Stock
Raw ---------------28----------------- (-21)------------ 4
We could have:
Product - Consumption - Overproduction Stock
Raw --------------28------------------(+31) -----------(-16)*
* = In 16 Wu's the rawhouse would be overloaded and then you pay more.