I usually find with treble clef, I am reading a melody in order to understand it and work it into the bass line. Therefore, I play it in whatever octave works, or usually I will play it both down low and up high to get it under my fingers. I know this doesn't answer your questions, but I guess all I am saying is that its generally immaterial to me what octave it 'should' be in, although I completely understand why you would like to know. Usually you can only play it in one place without going up to thumb position, due to low Eb's and their other sub-E brethren.