Grand total of a formula; Error: This field cannot be summarized

Hello all,

Beginner here.

I am writing a report where I have a weighted column of how long an invoice has been open. The column is called weighted and it takes from my invoice amount and the sum of days it's been since it has been open.

In my weighted column I am trying to have a grand total sum of this weighted value.

The current formula behind {@weighted} is: ({INVOICE_LEDGER_ITEM_CU_QRY.INVOICE_AMOUNT}/Sum ({INVOICE_LEDGER_ITEM_CU_QRY.INVOICE_AMOUNT}))*{@#Days}

I have tried the workaround of:

WhilePrintingRecords;

Numbervar x:=x+{@Weighted} //placed in group footer

and

Whileprintingrecords;  

Numbervar x;  //placed in report footer

This workaround just shows the last weighted value at 25.79.

Are there any other workarounds to get a grand sum of the weighted column?

Thanks,

Ajax