Public IsMileageCalculated As Boolean

Private Sub Form_Current()
    If Not Me.IsMileageCalculated Then Me.CalculateTotalMileage            
End Sub

Private Sub Form_AfterInsert()    
    Me.IsMileageCalculated = False
End Sub

Private Sub Form_Delete(Cancel As Integer)
    Me.IsMileageCalculated = False
End Sub

Private Sub Form_Dirty(Cancel As Integer)
    Me.Parent!tboMileage =  Me.Parent!tboMileage
    Me.IsMileageCalculated = False
End Sub

Public Sub CalculateTotalMileage()
    Dim rst AS Recordset
    Dim varTotalMileage As Variant

    varTotalMileage = Null
    Set rst = Me.RecordSetClone

    With rst
        Do Until .EOF
             'порой бывает не очень удобно писать в сумму ноль при отсутсвии
 
             'записей в подчинённой форме или при отсутствии суммируемых значений
 
             'по мне, так лучше если есть числа, то и в сумму пишем число
 
             'ежели нечего складывать, то и поле суммы ставим Null.
 
             'Это более информативно (IMHO!)
 
             
            If IsNull(varTotalMileage) And IsNull(!Mileage) Then
                varTotalMileage = Null
            ElseIf IsNull(varTotalMileage) And Not IsNull(!Mileage) Then
                varTotalMileage = !Mileage
            Else
                varTotalMileage = varTotalMileage + Nz(!Mileage,  0 )
            End If
            .MoveNext
        Loop
        .Close
    End With

    Set rst = Nothing

    If IsNull(varTotalMileage) Then
        Me.Parent!tboMileage = Null
    ElseIf 
        Me.Parent!tboMileage = CInt(varTotalMileage)
    End If

    IsMileageCalculated = True    
End Sub

Private Sub Form_BeforeUpdate(Cancel As Integer)
    Dim frm As Form
    Set frm = Me!subFrmControl.Form

    If Not frm.IsMileageCalculated Then frm.CalculateTotalMileage 
End Sub

If frm.Dirty Then frm.Dirty = False