Crystal - Cross tab - formula

[locheve]

Hello,

I have created a cross tab report showing promised dispatch dates of all our part numbers. The sales orders go out months but i have limited this report to 4 weeks. In the cross tab i have also added columns to show current stock available. I want to create a formula that subtracts the total orders to ship within the next 4 weeks from the stock on hand figure. The grand total column shows the total but i can't figure out how to use the grand total in a formula?

I don't want the results to appear in the cross tab, but i want to use the results to filter the report.

Regards.

 

[CoSpringsGuy]

What version of Crystal?
Can you provide a sample of what your output looks like via dropbox?

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Crystal Reports 2011 and XI
Intersystems Cache 2012 ODBC connection