The 1/3rd reserve rule of thumb is a good general rule, but clearly Kharon is right about variations thereon. Depth is a reasonable variation, unless the hazard for which you want a reserve is entrapment or entanglement, in whihc case you want all the air you can get. Tank size is a reasonable variable to use in deviating from the rule of thirds too, unless of course, you use a bigger tank because you use more air than others. 1/3 of a 100 may be the same time for the high rate user at 1/3 of 80 is for the more moderate consumer. The pointis that gereral rules are about as good as one can do, because general exceptions aren't much better. Now INDIVIDUAL rules work, and each diver can create them based upon logging a slew of dives, analyzing their air consumption under different conditions, depths, and with different equipmnet, and chart out their rules. Debbie and I have done this, to a degree, based upon hundreds of logged dive data. ( A reason to log dives, don't you know) However, for the casual recreational scuba diver who does10 or less dives per year, I recommend following the general rules as a nice safety margin. As for the intrepid diovers of great experience, I see no problem in deviation from the generalel rules which is consistent with your diving experience and track record. BTW, neither Debbie or I have ever ( and that's over 10000 dives, run out of air or come close, at least not too close.
DivemasterDennis