Inserting a running total from a Formula field to OE Invoice report

To all Sage300 Report and/or Crystal Report gurus:

I have an item optional field that holds a number value that I need to use to compute a quantity for each line item for which I need a running total in the footer in the OE invoice report (OEINV01.rpt). The Optional field table is not part of the Database fields for that report which uses OEINPR3.DLL.

What I did is create a subreport on table icitemo to get my optional field value. I created a formula field with a Shared variable to pass the value to the main report in the subreport.On the main report I created another formula field to use the shared value which I multiply by the QtyOrdered. Crystal Report is not allowing me to create a running total on that formula field.

Does anyone know how I can achieve a running total?

TIA