Hi,
Create a new Info structure with three key figures say..Sales, Stock, Production Plan...through MC21...
when you create a planning type for the above info structure create a Macro like this
Production = Sales -- Stock.
This will fulfill your req.
Process Flow....
Create info structure – MC21
Activate the info structure – MC22
Assigning the planning parameters to the info structure. – MC7F
Creation of Planning Hierarchy – MC61
Creation of planning type and Macros- MC8A
Calculate Proportional Factors – MC9B
Planning in the planning table – MC93
Copy planning version - MC8V
Transfer planning data to demand management – MC90
Check the below link...
Flexible Planning in SOP process flow...
Thanks
Kumar