Using the data source doesn't seem to work? What do you mean?

Formulas can be created by using a "Set a field's value" action in a rule to
set the value of a field to an expression containing a formula, or by using
an expression for the "Default Value" on the properties dialog box of a field.

To get an addition: field1 + field2
To get a subtraction: field1 - field2

Look up "formulas" and "expressions" in the InfoPath help file to get more
info.
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S.Y.M. Wong-A-Ton