Suggestion for WLM reading pane toggle

I find myself toggling the reading pane On and Off a lot. I've put a button
on the toolbar which makes this easy, but there's one wrinkle; when you
first turn it On it opens on the right, then on the second click it moves to
the bottom. I would like to be able to have an option to fix the pane in
one position (and ignore the other) so that the second click turns it Off.
Hope this is clear. Thanks!