A logic for Planned purchase order to use 0% sales order requirement and 100% demand forecast requirement

Hi, I have a special requirement for the master plan, and not sure how to do it in AX 2012.

I need to generate planned purchase order based on forecast only, regardless of sales order requirements. In other word, I need to reduce sales order to 0%, and use 100% forecast requirement in net requirement calculation.

So the logic I want is planned purchase order= BOM line requirement from explosion of (demand forecast for finished good requirement - production order open qty - on hand finished good) - on hand stock raw material - purchase order open qty

In this scenario, whatever reduction key I use, coverage group setting and reduction principal I set in MP, it doesn’t work.

Any one knows how to realize such logic in AX 2012 MRP?