Yes/No Field not to add tax

[Bob V]

On my Form I have created a Yes/No Check box , What i am trying to do is
stop tax being added to any amount in that record that has the check box
ticked
I will have to alter my code, Any help with this would be
fantastic.......Regards Bob..here is a sample of some of my code
My Control is called [NoTaxAdd]
So it would be something like , SubTotal will not add totals
(checked)[NoTaxAdd] and [TotalAmount] will add amounts (Checked)[NoTaxAdded]
tbNoTaxAdd.value =.Fields("NoTaxAdd")................Am I going down the
wrong path!!!!

Private Sub subShowInvoiceItMdtValues()
subBlankForm
With recInvoice_ItMdt
tbIntermediateID.value = .Fields("IntermediateID")
tbInvoiceDate.value = .Fields("dtDate")
cbHorseName.value = .Fields("HorseID")
tbFatherName.value = .Fields("FatherName")
tbMotherName.value = .Fields("MotherName")
tbDateOfBirth.value = IIf(IsNull(.Fields("DateOfBirth")), 0,
Format(CDate(Nz(.Fields("DateOfBirth"), 0)), "dd-mmm-yy"))
tbSex.value = .Fields("Sex")
cbGSTOptions.value = .Fields("GSTOptionsText")
tbGSTOptionsValue.value = .Fields("GSTOptionsValue")
tbSubTotal.value = .Fields("SubTotal")
tbTotalAmount.value = .Fields("TotalAmount")
End With
End Sub