First off, you probably would have gotten quicker response had you posted this in the Reporting Services forum.

Your expression sounds fairly simple to me, if I understand correctly. In the textbox where you want the expression (or in computed field in the data set) set the formula to: =Volume/(FTE + Over 8 FTE). Based on what you are saying they are all at the same group level. If they are at different group levels you can reference the textbox values using the ReportItems collection. I can't remember the exact syntax, but you could use BOL to find the syntax. It would be something like: =ReportItems!VolumeTxtBox.Value/(ReportItems!FTETxtBox.Value + ReportItems!Over8FTETxtBox.Value)