CKF PO Unit/Cost {On Order=0}

Hi Team,

I have to write a logic for On Order

In my report I have created two CKF's for PO Cost and PO Unit. then I have created the 2-RKF's (On Oeder Cost/Quantity) and restricted some values (0DOCTYPE) 

CKF= PO Cost =PO Cost- Goods receipt cost

CKF=PO Quantity =PO Unit Qty - Goods receipt Qty

As per the logic I need to show the report and we need to revise the On Order logic to say if greater than PO Unit/Cost then On order = 0

Ex : PO Cost   Goods Recpt Cost    On Order Cost

         11                         9                          2

          9                          12                        0    (Correctly I am getting -3)

Can some one please help me..

Thanks

Prakash