For open water ascent only: take your SAC rate and your buddy's sac rate and add them together (0.6 + 0.5 = 1.1). Take the depth that you're going to start your ascent from, cut that in half and translate that into ATA (66 FSW/2 = 33 fsw = 2.0 ata). Take the same depth and translate it into minutes at the ascent rate you use (66 feet @ 30 FPM = 2.2 min). How much air will you need for your ascent? 1.1 cfm * 2.2 min * 2.0 ata = 4.84 cubic feet). That's bare bones, no room for excitment (double it for that later) no room for a safety stop (which at 30 fpm is really not necesscary and which in an emergency you're going to blow off anyway). So I'd round the 4.84 to 5.0 which translates to a bit less than 200 PSI. So multiply in by the excitement factor of two, and you're at 400 PSI. Now that's just your ascent based on those SAC rates, figure that you never want to drop your tank below 200 psi ... esp. if two people are trying to breathe off it, and now you're at 600 psi for these two divers starting their ascent from 66 fsw.
Bingo air (rock bottom) or turn point is a completely different discussion.