How to Minus one Running Total column from another inside Pivot?

Hi, In my pivot chart I want a column that shows the difference between a cumulative budget and cumulative actual column. I have created these cumulative value columns by using the Field Setting/Use data/'Running Total In' facility. However when I try and use the Calculated Field function so that I can minus one field from the other these fields are not available in the list to choose from. I assume Excel does not see them a real fields. How can I get around this?

My Pivot should look like this

Month Actual Budget Cumulative Actual Cumulative Budget Cumulative Difference

Jan £1000 £500 £1000 £500 -£500
Feb £1250 £2500 £3250 £3000 £250


Thanks

Excel 2010 with free PowerPivot Add-In
All of the previous Calculated Fields/Items shortcomings have been fixed in PowerPivot.
http://www.mediafire.com/view/plc49haxq2wj9tk/10_02_14.xlsx

Add-In is at http://office.microsoft.com/en-au/excel/download-power-pivot-HA101959985.aspx