Pivot table - There are too many records to complete this operatio
 

I have created an Excel pivot table from a SQL Server data source. The pivot
has columns showing "Actual" against "Forecast" numbers for each department.

The problem is, when I attempt to add a calculated item to show the
"Variance" which is = "Actual" - "Forecast", I get the message
" There are too many records to complete this operation".

For testing purposes I reduced the dataset to just 10 rows but still got the
same message.

Any ideas will be appreciated.