I have a matrix report that shows Count & Average as columns against data (this is a ReportServer usage report). The issue is that the Average column should be calculated as UsageCount/NumberDays in the detail area (where number of days is the number of working days in that month), but in the sub-total area it should be calculated as the UsageCount/TotalNumberDays (where this number is the total number of working days included in the report).