Where and how to enter an Excel formula

[Bob Phillips]

Private Sub Worksheet_Change(ByVal Target As Range)
    Const WS_RANGE As String = "A:A" '<== change to suit
    Const ADD_FORMULA = "=IF(M<rownum><>"""",""Returned"",IF(AND(TODAY()>=K<rownum>+8,M<rownum>=""""),""Require Chasing"",""No Action Required""))"
    On Error GoTo ws_exit
    Application.EnableEvents = False
    If Not Intersect(Target, Me.Range(WS_RANGE)) Is Nothing Then
        With Target
            If .Value <> "" Then
                Me.Cells(.Row, "N").Formula = Replace(ADD_FORMULA, "<rownum>", .Row)
            End If
        End With
    End If
    ws_exit:
    Application.EnableEvents = True
End Sub