A lot of great insights here. To address the question of EXACTLY how much air should remain in the tank by the time you reach the safety stop, here's what I do.
I want my divers to be at the safety stop with 50 bar. The characteristics of the dive site might make a difference in what we consider low on air.
When I take divers to the King Cruiser wreck, for example, I have a low-on-air rule of 70 bar. Why? Because we're doing essentially a square profile dive at great depth. Just getting back to the line and going up it takes a while, and by the time we get to the 5 meter stop, most divers who began to ascend with 70 will then have 50 bar anyway. Add the safety stop time, and many will get on the boat with 30 left in the tank. I use a 50-bar rule for multi-level diving conditions.
When I'm leading reef dives, I always ask divers for their SPG readings throughout the dive so that I can manage the dive to get the longest dive time from a tank. If I know when people are at their halfway point (100 bar), I can almost always get the group progressively shallower in order to extend the dive to the max dive time, even with big European divers who literally hoover their air.
It was noted by somebody that PADI doesn't teach the rule of thirds. This is true, but we do teach the effects of depth on air consumption and we teach monitoring of consumption. With adequate training, divers can do for themselves what I do for a group: that is, monitor their consumption and dive ever shallower so that they can get the most diving time from their single tank.
IMO, the rule of thirds is too simplistic for multi-level diving. It assumes a square profile and out-and-back nav. For the kind of one-way-get-picked-up-by-the-boat diving on sloping reefs that we do around here, it's much more effective to monitor consumption. As noted above, a diver can get fancy and pre-plan depths on multi-level dives according to RMV calculations, but it's not really necessary. I don't have these calculations for the divers in my groups when I lead dives, and I still manage to get them to their safety stop at 5 m with at least 50 bar in their tanks. The only times this has failed and divers have come up on my octo have been when divers either can't see their SPGs and misread them or alternatively lie to me to save face (yes, incredibly, this does happen).
