Calculated field question

I have a query that summarises sales data for a particular month and cost
centre. The data comes from our accounting system and is held in a single
table in my database thus:

tblNominal
------------
CostCode - Long Integer
ExpenseCode - Long Integer
Desc - Text
Current - Double
YTD - Doube
Month - Text
Year - Text

The query sums the Current & YTD values for a particular month, grouping
them by the Description field. This works fine to sum the Sales for say
Retail (which is a group of Expense Codes) but I also want the query to show
the Margin for that type of sale in a neighbouring column. This value is not
held in the table, but could be calculated as there is a group of codes
having the description Retail Cost of Sales, and Retail Margin would be
Retail - Retail Cost of Sales. Is this possible? What would be the best way
of going about this?

Thanks
Martyn
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Access 2007, Windows XP