I agree with geralf. It is almost certain that you do not need to store the calculation. You can do the same calculation in a query so that it is available whenever you need it to be. That way, you don't have to worry about any of the fields involved in the calculation being changed and the calculation not being re-done.
However, to save a calculated value to a table column does not require that the bound column appear on the form as a control. It simply requires that the column be present in the Form's recordsource.
Me.YourFieldName = Me.YourCalcControlName