Interesting read ... for those of you attacking Lamont, the rule of thumb wasn't his idea. It's been around for a long time ... I've also included the same rule of thumb in my
gas management article ... which was written long before Lamont's. It appears in several others that I'm aware of.
The origins of the "rule" predate my own OW class in 2001, and it was intended FOR NEW DIVERS ... folks who typically have a very high gas consumption rate and who generally don't know much more than "watch your SPG" and "end your dive with 500 psi". The purpose of the rule of thumb was to give these divers something tangible that could possibly help keep them out of trouble ... because here in Puget Sound, where AFAIK the "rule" originated, it is quite easy to get stupid deep on a simple shore dive ... and every year we have at least one fatality due to someone going deeper than they should and running out of air.
The rule was never intended to apply to more experienced divers, because those divers (a) develop lower consumption rates with experience, and (b) develop enough knowledge about how to manage their dive to make better judgment calls concerning depth and available gas supply.
So please ... keep things in context. Scuba instruction is chock full of rules of thumb. They're mostly designed to provide a simplified framework for the new diver developing basic skills to keep them out of trouble while they develop enough of an experiential context to use good judgment.
That, really, is all it is.
Oh ... and getting back to something the OP said ... you are correct. I NEVER allow my AOW students to do their deep dive on an AL80. Prior to doing the dive, we do gas consumption measurements, and I provide them a dive plan for the dive. Using their own measured gas consumption and the dive plan, I have them calculate how much gas they would need for that dive. Without exception, every student who has ever taken the class has concluded that an AL80 is too small of a cylinder for that dive. Oh, many won't use the 77.4 CF of gas contained in that cylinder ... but when they factor in how much they would need for a controlled ascent while sharing it with a buddy, they quickly realize why it's inadequate.
The minimum size cylinder we use for the AOW deep dive is an LP95 ...
... Bob (Grateful Diver)