Step 1: On Format Designer > Right Click and Add Calculation Field on OutstandingSO. 

Step 2:  Right Click on Calculation Field and Edit Calculated Fields.

Step 3: Modify Calculation Field as below:

  i) (Name) = OustandingAmt

  ii) Data Source = OutstandingSO

  iii) Expression = ([Net Amount] / [Qty]) * [Outstanding Qty]

Step 4: Pull OutstandingAmt field into format

Step 5: Change Format String for OustandingAmt field =  {0:n2}

Step 6: Save the format and preview. Outstanding Amount will calculated.