MES - End Activity Screen (Epicor 9.05)

We are attempting to capture data on the number of manufactured items which Pass or Fail our 1st quality inspection process within our manufacturing flow.
I have added two numeric fields to the End Activity screen within MES, entitled Pass Qty and Fail Qty.  I would like to verify the quantity entered into the "Completed" field on the End Activity screen, is the sum of the values entered into the Pass/Fail quantity fields.  The formula is {Completed Qty - (Pass Qty + Fail Qty)}.  If the equation results in anything other than 0 (zero), I want an error message to display, which states the quantities entered do not balance / equate to the quantity Completed.
Using a In-Transition Data Directive, when the error message displays and the user clicks OK, the End Activity process completes and MES does not allow you to correct the mistake. 

Originally, I thought I would need to perform the calculation on the actual End Activity screen, using the Customization tools.  I could not get that to work the way I wanted; when the screen loaded, it indicated there was an error due to the fact the formula did not equate to zero, and I could not get the screen to only perform the validation after the <OK> button pressed.

How can I get the End Activity to re-display the screen when the formula does not equate to 0 (zero), so the mistake can be corrected?
Should this be on the End Activity screen with Customization or can I use a Data Directive and redisplay the screen if an error exists?

Any help is appreciated.
Thanks,

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Glenn Owers
ERP Business Analyst
Safran Electronics Canada
Peterborough ON
705-743-6903 x231
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