% of Total Calculation

[stevekoz]

Playing in a crosstab report in cognos analysis studio and i'm trying to create a custom calculation.

My cross tab looks at Volume sold of item by location by year broken down by month and also forecast sale volume by same cross section -


                                                 Col A                     Col B           Col C                Col D                  Col E
                                            Forecast Value |    Product A    |  Product B    |  Product C |     Total All Products
Location A | Month A |Yr             123                    123                123                  123                   12345

I want to calculate the forecast volume as a % of Total. If i were doing it in excel it would be something like

=(E1-A1)/A1

Is there anyway of writing a custom calculation like this ? 

Cheers