Rules of Thumb
Warning
These rules of thumb are not the whole story. They are simplified as much as possible to make them more widely accessable. As with all massively oversimlified rules they may not apply well in every circumstance and are necessarily aimed at the worst case of beginning divers and harsher conditions. They are hopefully simple enough for all divers to be able to employ with minimal math and should be able to be recalled even through the haze of narcosis.
Tank Size
The rule of thumb is that you should not dive to a greater depth in fsw than your tank capacity in cu ft. This works reasonably well for most beginning and intermediate recreational divers and caps the depth that an Al80 should be dove to to 77 fsw (Al80 == 77.4 cu ft). A steel 100 tank should not be dove past 100 fsw and a steel 130 should not be dove past 130 fsw (which is the limits of recreational diving as well).
In some circumstances, experienced divers in good viz and warm waters may do dives to 100 fsw routinely on Al80s so this rule may clearly be stretched.
It is probably not a good idea to be doing dives to 130 fsw on Al80s under any circumstances, though, and inexperienced divers (100 dives or less) doing coldwater dives to 100 fsw on Al80s are what this rule is squarely aimed at preventing. Also, the diver with 100 dives who think they're okay with an Al80 at 100 fsw in warm water and good viz is probably at the edge of being overconfident.
Simplified Rock Bottom Values
For HP120/HP130/LP104/LP95s (big tanks) use a rockbottom value equal to 10 times the depth in fsw that you are at. So, for an X8-130 high pressure Worthington tank at 110 fsw, you must leave the bottom with at least 1100 psi.
For Al80s/HP100s/LP80s/LP72s (small tanks) use a rockbottom value of 10 times the depth in fsw plus 300 psi. So, for an Al80 at 60 fsw, you must leave the bottom with at least 900 psi left in the tank.
For both of these rules, never use a rockbottom value of less than 500 psi. Be aware that this simplified extrapolation breaks down as you go deeper. For an E8-130 at 130 fsw the appropriate rock bottom value is closer to 1600 psi. If you are diving deeper than 100-110 fsw you should be able to follow the full rockbottom and SAC rate calculations below.