we need to make some calculations on summarized data in reports. such as summing up two or more column values or dividing two column's values in each other to get a percentage value from them. Calculation have to be functional not only in data rows but also in summary rows in a report.
My suggestion is to make a calculated column that is similar to pivot calculated area which is enabled in excel sheets in pivot reports.
I think, this will increase productivity of reports and gives more flexibility to users.