Roll Up Filtered Detail Group

I the following report with one group:





Month Number of Sales Total Sales



+ Jan 2007 100 $1,000.00



When you dril down on MonthYear you get the detail data:





Month Number of Sales Total Sales



- Jan 2007 10 $610.00

1 $10.00

1 $20.00

1 $30.00

1 $40.00

1 $50.00

1 $60.00

1 $70.00

1 $80.00

1 $100.00

1 $150.00



My question is. I added a filter to the detail data to give the Bottom % =75 of sales. So My detail data only displays the following rows:



Month Number of Sales Total Sales



- Jan 2007 10 $610.00

1 $10.00

1 $20.00

1 $30.00

1 $40.00

1 $50.00



My problem is the group still displays the total of my dataset (as seen above), but I want it to display the total of the detail data group, like below:



Month Number of Sales Total Sales



- Jan 2007 5 $150.00

1 $10.00

1 $20.00

1 $30.00

1 $40.00

1 $50.00



If I change the fields in the group to look at the detail data ,for instance =count(Fields!NumberofSales.Value,"Details_Group") I get an scope error.



How can I display the totals of the detail data in the parent group after I added a filter to the detail data?