Validate two sums and display total

I have a form which sums numeric data (amounts) in both columns and rows. I need to verify that the sum of the row of totals and the sum of the column of weekly totals are identical and dsiplay the total in a single grand total field. Because the form was imported as a fixed field form Tables and Subforms are not available. I've set up individual fields for AllTotals and the WeeksTotal elsewhere on the form that I can make invisible thinking I could use an if expression but I can't figure out how to display the sum in the GrandTotal field. Any suggestions?