How to create a “production plan” when using ASCP??

We have an operational model, and ASCP plan, which has multiple Distribution Centers ‘DCs’ and multiple Manufacturing plants.  DC’s are restocked from plants through the standard order Internal requisition / Sales Order process via sourcing rules / assignment sets that “transfer from” the plants to the DCs to meet demand.

The issue we are struggling with is the inability to meet forecast demand, i.e. the forecast calls for qty 100 for item A per month, but the plant which makes A can only provide a maximum of 40 per month (sometimes less).  So, currently we have forecast of 100 from DC 1 to Plant 1 which requires DC 1 to manually intervene in reducing the forecast demand to a “production plan” for Plant 1.