Does RDLC calculate current totals?

I'm struggling to get the current amount to work on the report. I want the following:

Product    Qty1   Qty2  Total  RunningTotal
ABC123     100    200   300    300
DEF321     50     100   150    450
XYZ123     -100   -150  -250   200

A common column is an expression like

=ReportItems!Qty1.value + ReportItems!Qty2.Value

To accomplish the total, I tried many different things, most of which tell me that it should be in the header or footer. An explicit expression that I thought would work,

=ReportItems!Total.Value + previous(ReportItems!Total.Value)

but it does not work.

Can someone explain what I'm doing wrong here?

Cheers Dean