While diving, you should always, at any moment have enough gas to make an ascent to the next source of gas or to the surface while sharing gas with someone. Same idea of Minimum gas on SubSurface planner.
It only takes 4 configuration parameters:
SAC
Tank Volume
Time to solve problem
Stress Factor
Many people dive consistently with the same tank size, so this would be almost a one time configuration and you get the Rock Bottom Gas at any depth at any time throughout your dive!