Excel 2007 : Pivot table calculations

Hi all,
I have a large data table which I'm doing a Pivot on that now would really benefit from some additional calculations. The problem is that I cannot seem to get it to work because the values I want to use in the calculation is in the same field.
For example I want to compare the Net Revenue between two periods to show growth in %:
Field 1= NR
Field 2 = Period (i.e. months)

I would then like to use a SUMIF or IF formula to be able to sum up all products Net Rev. in Period 2 (Feb.) and compare to Period 1 (Jan.). Normally I'd SUMIF Feb. and divide by SUMIF Jan to get the desired result, but it's not so optimal to use a column outside the Pivot table if I can help it.

The data is also a refreshable Query so no good to mess with the table structure I'm afraid.

Does anyone know how to solve this?

Best regards,
/D