Calculated Value In A Text Box To Be Stored In Table

I am using the following and is working perfect

Private Sub NEXPPRC_AfterUpdate()
Me.[Purchase Description] = Me.NEXPPRC.Column(0)
Me.[Purchase Price] = Me.NEXPPRC.Column(1)
Me.[Purchase Units] = Me.NEXPPRC.Column(2)

[Code] .....

The above is for combo box, Now I want to use similar code to store calculated text in a field to the table "Purchases" but for the text box

Name of the text box:
Atcom
calculation Value:
AASTamt: ([aasc]*[Purchase Qty])

I tried but this is not working on the text box Atcom

Private Sub Atcom_AfterUpdate()
Me.[Atcom] = Me.([aasc]*[Purchase Qty]).Column(0)
Me.Requery
Me.Refresh
End Sub