Creating Grand Summary From Calculated sub Summary

I have 2 tables one containing stock items and one containing Stock Requirements.

They have the following fields:-

STOCK

Stock_code (number auto enter serial)

Qty_in_stock (number)

Qty_on_order (number)

Cost Price (number)

Stock Required

Part_no (text) related to STOCK STOCK CODE

Count_of_part (Summary = count of Part_no)

Qty_Required (calculation Case(GetSummary ( CountOfPart ;Part_no )-Stock::Qty In Stock-Stock::Qty On Order ≥ 0;GetSummary ( CountOfPart ;Part_no )-Stock::Qty In Stock-Stock::Qty On Order;0))

Total_item_cost (calculation qty_required * Stock::Last Cost Price)

This all works and allows me to use a layout with a sub sumary showing a breakdown of stock_code, qty in stock, qty on order, qty_required, cost price and Total_Item_cost.

What i can't seem to do is then get a grand summary of total cost.

I have tried a field of total_cost summary = totoal of total_item_cost but this gives the totals of all the total_item_cost data and not a total of the summary.

how can i get a total of the summary