lamont:
100 psi per every 10' of depth plus 300 psi pretty much duplicates my numbers.
my numbers are based on an OOA scenario with a combined SAC of 2.0 for both divers (your buddy who is fearing for his life may have a consumption above 1.0) and 1 min at depth, 30 fpm ascent to the surface and 1 min @ 30, 1 min @ 20, 1 min @ 10 for stops.
Sometime back I posted how I came up with the numbers I use ---- One scenario is normal ascent of just me, with a 20 to 30fpm ascent rate, and very generous set of stops and 500psi left in the tank. For the "share air with a freaked out buddy" scenario I am willing to do 30-50fpm, blow away the non-mandatory safety stops and accept the very slightly higher DCS risk, and don't require an extra 500psi in the tank upon surfacing. With a panicked diver I'd get within 15' of the surface before deciding on whether to go on up or hang for a while. Both sets of calculations came out pretty close to 1000 psi at 100', 700psi at 70'.
As for rule of thirds, you need to figure out just what is truly required and what your are willing to blow away as a requirement. One basis for the rule of thirds is that if you go into a cave using 1/3 of your gas, and then you or your buddy loses all of your gas; that you will have just enough to get back out if you each use 1/3 of your gas while sharing air back out. "Rock Bottom" calculations include consideration for an air sharing ascent and are more appropriate for non-overhead envirionments.
Even on a non-deco rec dive you may effectively have an overhead and need to account for it in your rock bottom calculations. For example, is your ascent pressure what you want at the upline on a wreck, or is it your minimum for starting an ascent from whereever you are at that moment?
Is your turnpoint pressure on a shore dive what you must absolutely have to be able to swim under a busy traffic lane, or is it just a case of being more convenient to return to shore underwater than with a long surface swim.
As a general rule, I set my turnpoint or ascent pressures such that, if it's just me using the air, then I have a nice, easy, elegant conclusion to the dive. OTOH, for the case of a catastrophic reg or tank failure that forces air sharing, I'm willing to accept the reasonably safe, although not optimum, dive conclusion --- such as doing a free ascent from a wreck rather than going back to the upline; accepting a long surface swim back in rather than the more convenient underwater return, surfacing in an area of boat traffic that I would normally avoid, etc.
The key thing is to think about different potential situations and what you need to safely end a dive with acceptable risk.