It is very seldom appropriate to store Calculated Fields; they should simply be re-calculated, as needed, for other Forms, Queries, Reports, etc.
The Expression Builder places the Calculations in the Control Source of a given Textbox. A Control Source can be populated with an Expression, or with the name of the Field the Control is Bound to, but it cannot do both at the same time!
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The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007