I have a form where the user enters BeginMileage and
EndMileage. After the EndMileage is entered, the
difference is calculated and entered in a subform control.
How should I handle the fact that the user may change the
BeginMileage. In other words, if the BeginMileage is
changed, I need the difference calculated again.

Thanks,
New to Access, BB

You shouldn't store the calculated value, retrieve it via a query or a calculated unbound control.