Calculated Key Figures based on Characteristic value

We are replacing Congos Impromptu with BW and BEx and have come across an issue with a calculated key figure currently being used in Impromptu that we need in BEx

The scenario is for calculating an outstanding order qty based on the qty allocated, the qty shipped and the value in the Material Catagory InfoObject.

The formula required is:

if ((qty_invoiced < qty_shipped) and item_category = 'TAS') then Outstanding_Qty = (Qty_Ordered - Qty_Shipped) else Outstanding Qty = 0

I'm relatively new to BW and BEx query writing, can anybody assist me with the above scenario.

Regards,

Mike