I have had the same reasons for storing a calculated field. The derivation of the final result is pretty complex such that it slows down my application to calculate this particular field when doing searches or reports ...
Create a field for the calculated control in the table the form is based on. Set the control source of the calculated control on that field. Most critical, before exiting the form (however you do that) ensure you do a save action so the calculation (or, in my case, the updated calculation) is saved into the field.
-dK