Help - Need to run variance calculation in Pivot table

I have a pivot table that accurately reflects actual spending and forecasted spending by account for a series of organizations/departments. I am finding it difficult to insert a calculated field that will show the difference (variance) between the actual and forecast for each month.

The table is laid out as follows – I need to add a column under each month (same level as type) to run this calc.

Month : Type(Act & Fcst)
Organization
Group
Dept

Month and Type were supposed to be indented in the message I posted to indicate they are in the columns of the table.

I can get the table to calculate sum, product, std dev, etc. but no variance..

please help!!