Calculated controls - How do I update underlying table with a calculated figure?

[ahdkaw]

I have a Purchase Order form which has a subform for Purchase Order Details. I can pass the Totals from the subform to the parent form, but for some reason I can't update those totals in the underlying Purchase Order table, as when a calculated control is updated it doesn't seem to trigger any events in the form or in the control itself!

I've tried changing to an Unbound form and updating to the tables afterward, but due to the PUOrderID being an autonumber, I can't use it as a reference to the Purchase Order Details subform without first updating the Purchase Order Table!

It would be preferable if I could do it as below (a BeforeUpdate on the control doesn't work!):

Sub CalculatedControl_BeforeUpdate()
UnderlyingTable!TotalValue = CalculatedControl.Value
End Sub

Anybody help? I hate order forms! Thank you.