During class instructions when we teach our students to plan their own dives we do teach this "rock bottom" theory. That being, if your SAC is X and you want to dive to Y, how long can you stay? The student manuals cover this topic and discuss that for every 10ft you are down you will need to keep a reserve of 100psi.
Hold on a sec. That's not "rock bottom." Basically, rock bottom is the amount of gas you keep in reserve so that, given your current depth, you can ascend with yourself AND YOUR BUDDY breathing off your tank(s), at a stressed SCR, while maintaining all of your safety stops. This reserve is "walled off" and is not to be included in your dive plan. So if you plan to use half your gas going out and half coming back in, you calculate your rock bottom, reserve it, and use only the remaining gas to determine halves.
Bob's site above (NWGratefulDiver.com) is truly a great resource for anyone who wants to learn more about gas management, as is Lamont's description (Rock Bottom and Gas Management for Recreational Divers).
As an aside, the 100psi for every 10ft doesn't take into account different tank sizes, and is in many cases is insufficient to serve as rock bottom.