Linking and filter a sum of a field

I am struggling with developing a report. I am not sure exactly how to explain it best but here goes!

We have orders that sometimes come in as blanket orders. We would like to be able to see those orders on a report, even if we have not received any releases yet (we are a processing facility, we set up the orders when we get the POs but do not release anything on the order until we actually receive the parts to process from our customer). The way I need to filter the report is to compare the blanket_qty in the co_bln table to the sum of the qty_shipped in the coitem table. The blanket_qty and the qty_shipped values should both come from the same co_num.

The formula I have come up with thus far is below. It returns a lot more values than I need as it is comparing a single value of the qty_shipped to that of the blanket_qty. I am not sure the correct process to get the sum of qty_shipped.

Any help resolving this issue would be greatly appreciated.

=NL("rows=2","coitem","co_num","link=","co_bln","co_num","=co_num","blanket_qty",">qty_shipped")
Mark