As far as I am aware, there are no rules- it is as far as the designer wants. I think you will find that with all widebodies with some sort of lower deck accommodation of any sort, it must be unoccupied for take off and landing, so the only things the designer will be concerned with are ensuring that adequate ground clearance is provided for flaps and engines and aft fuselage to ensure no scrapes. So the BAC1-11, DC-9 and BAe146, Challenger etc all have very low fuselage/ground clearance. The widebody ground clearance is determined by engine/flap and tailscrape considerations. Galaxy, Starlifter Hercules etc show that when those factors aren't so critical, you can give minimal fuselage/ground clearance.
I think you will find with the Tristar lower deck, it too had steeply sloping sides. They had a lower deck galley, but if you were going to give more of the lower deck over to passenger accommodation, there would be too little space left for baggage and cargo, and this was probably the constraining reason.
Last edited by Rainboe; 17th March 2006 at 11:28.