Change PowerPoint Notes Frame Size in Normal View

How can I programmatically change the position of the slidebar that appears between the PPT slide and the notes below the slide, using VBA? (the slidebar highlighted in the red box in the image below)

Searching around on the MSDN, I can't find any property that seems to control the position of this slidebar. However, when I drag it to a new position and save my presentation, the position of the slidebar is saved, so I presume there must be some way to set it.

It's stored in the property SplitVertical on the DocumentWindow object

Sub Test()
    Dim app As Application
    Set app = Application
    Dim oWindow As DocumentWindow

    For Each oWindow In app.Windows

        Debug.Print oWindow.SplitHorizontal
        Debug.Print oWindow.SplitVertical

    Next oWindow
End Sub