Makro auf markierten Bereich ändern?

Sub Index()
Range("G5:G20000").NumberFormat = "@"
For i = 5 To 20000
If Cells(i, 12).Value = "Import" And Len(Cells(i, 6)) = 10 Then
dummy = Cells(i, 6).Value
Cells(i, 6).Value = Left(dummy, 8)
Cells(i, 7).Value = Mid(dummy, 9, 2)
End If
Next i
End Sub