Hi David (and Gayle),
No such "break point" option exists currently (and never has). It is a "wish list" item to add that option, but the current algorithm actually is much more complex than a simple "all notes above X are right hand, all notes below are left hand". Typically Composer does a pretty good job of analyzing hand movement and placement and thereby putting the notes on either the treble or bass staff accordingly, but it's still (obviously) not perfect
ttfn,
Sherry