David,

Sorry I misunderstood your problem, I thought you were getting the total of all rows and wanted just the total for the filtered rows. Yes, what you are doing should work, but I would recommend pushing that down to the query by adding a column doing a sum(totalturnover) over() which returns the sum of all the rows in the results. Then in the total row on the report you'd use first(totalturnover) instead of sum. I'd do that because I find it easier to understand and maintain than hiding textbox and then referencing it in anther object in the report. Both work so it really comes down to personal preference.