Calculated subtotal

[DAN]

Apologies is this is a silly question, but I'm new to Cognos (using 8.3) and am really struggling...

I have created a simple report that displays the costs (data item) by period (column header, but with separate data item for each month), grouped by cost area and cost type (Row header), with totals for cost area and cost type.  The report uses filters via a prompt page also.

The issue is that I would like to have a subtotal line based on combination of some cost areas and some cost types, effectively a calculated total. 

This is the method I followed:
1.  Create a new data item for this, (using a case when then based formula to get only the costs required)
2.  Add data item to existing report
3.  Create total for data item in header
4.  Delete data item rows to leave on the header containing the calculation.

However the header containing the calculated total repeats for each cost group.

Does anyone have any ideas on how i can position and control the total header, or is there a better/alternative approach?

Thanks in advance
Dan