PowerPivot add calc item to a field

[Guenh]

Hi, I have a data in an table stored in an excel file. I linked this table to PowerPivot Data Model and from that Data Model I want to create a Pivot in the same Excel file. In this table one column contains split of data into: Budget, Last Year, Prior Forecast, Current Forecast. I want to add this field as Pivot coulmns, but I would like to add additional calc item (like in normal Excel Pivot Table I can add Calculated Item) with calculation: [Current Forecast] - [Prior Forecast]. I checked various pages, forums, etc. and I have not found any guidence on how to add such calc item to afield in PowerPivot. Maybe you can help me, because this issue is in my head for over a week and is really my pain in the a*s now and I can't find any answer for that...