Here is a sample from the CR .Net API reference. The idea is to create an empty formula box. Then use this method to pass in a value. You can reference the formula field by its name as a string and not its index value. I do this when I have some dynamic value I create in code that is not in the database but need it to show on the report.
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Private Sub UpdateFormulas(ByVal myReportDocument As ReportDocument)
myReportDocument.DataDefinition.FormulaFields.Item("myformula").Text = "{file.SALES} *.1"
myReportDocument.DataDefinition.FormulaFields.Item(2).Text = "{file.SALES} + {file.COMMISSION}"
End Sub
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Hope this helps.
Regards,
John W.