Are you using Tab 6.0? If so, you should be able to use the new Window_ Calculations to accomplish this.
You have a few options, and it depends on how you want to display the final numbers, but this may get you started:
1. create a calc field like:
2. right-click your "SUM(Quantity)" green pill, and select Quick Table Calculation -> Running Total
3. this will add a triangle to the pill, and when you right-click it select "Edit Table Calculation"
4. change "Running along" to "Advanced", and set the options as in the attached image (the order of the fields in the list box is important)
That will get you what I did in "Sheet 2" in the attached workbook.