DA Aquamaster:
You have the math right and more importanlty you recognize that a certain psi in one tank will not represent the same volume in another type of tank. Many divers never even figure that out.
As to whether a certain reserve is enough gas or not depends on the specific situation. I would argue thaqt 500 psi is more than adequate in many recreational situations (shallow no deco dives in non over head environments with ample buddy support) and I would also argue that in some situations a 1/3 reserve may be far less than adequate (deep deco dive in an overhead environment with a hoover buddy that you may have to support on the way to the surface or first deco stop and/or in the event you lose one or more of your deco gasses and have to complete the deco on back gas.)
The important thing is that you understand the relationship between psi and volume and can work the problem either way to identify what psi you need to retain a certain volume in reserve.
The important thing is that you reserve enough for you and a budy to get back, up or out whichever is appropriate.
With identical sized tanks it's just a matter of determining turn/ascent pressure.
With different sized tanks, you calculate the turn/ascent pressure (could be thirds quarters sixths or whatever) for the small tanks and then the person with the larger tanks limits themself to a volume equal to that as usable prior to turn.
When done that way you are assuming that the same amount of gas will be used by each diver on the way out as on the way in but it doesn't matter that one diver consumes more than the other. Compensation is built in because each diver is allowed the same amount. When it's gone, you turn and as long as no one cheats on the turn pressure ther will be enough to get them out if you need to share. Of course if there is trouble and you're sharing gas, breathing rates may very well go up and swimming speed may be different which is one reason that 3rds is the most liberal gas plan used for overhead dives.
The gas needed for ascents and decompression is different in that the ascent is time limited and not necessarily the same speed (almost always slower) as the descent so breathing rates do come into play. The GUE rock bottom that's taught in the DIRF for recreational no-stop diving assumes that 1 cu ft/minute is adequate for each diver even under stress. That's less than some divers (a minority) will use but the stops they have figured in are just safety stops so they could be skipped and you could bump up the ascent speed a little. In either case, for this reason, the back gas needed for ascent/decmpression is backed out prior the turn volume calcs (3rds or whatever) on overhead dives.
Why worry about all this for recreational dives? Because some recreational dives are still deep enough to require a significant amount of gas for the ascent and in some cases it's very desireable (even mandatory) to get back to the entry point before ascending (maybe because of heavy boat traffic or something. The techniques for making sure that there's enough gas to do that is exactly the same as it is for a big cave dive or a deep staged decompression dive only without as many stops to calc individually (thank goodness for software).
