Well, the easiest way to determine bottom time/air consumption from previous dives is:
find a dive that was at the same depth throughout.
Take the tanks pressure rating (AL80 @3000psi) divide the 3K by the size. This gives you .026666 cuft/psi.
Then, looking at your log, find a start pressure and end pressure (all to a consistant depth). Subtract-this is the psi used.
Then, Multiply this by the cuft/psi (.0267) This it the cuft used.
Take this number and divide it by the time of the dive. I would suggest lowering the amount of time by 5 to 8 minutes to allow for descent/ascent, etc, this will give you a more realistic and answer. This is the cuft/minute
Then take the depth, divide by 33, add 1 to the answer, and you have the surface air consumption rate.
Now, as long as your 72s are filled to 2250, simply multiply this times 72, and it gives the amount of time you could stay, sorta. You really don't want to base a dive plan on this, because, that would leave 0 psi in the tank for one, and you don't know how conditions are going to be, stress and other factors use air a lot faster. In order to get a true idea of you air consumption, go to 33 ft, rest for a few minutes, mark the pressure in the tanks, relax for say 4 minutes exactly, mark the time/pressure, then do a swim at 33, marking time and pressure, then swim as hard as you can hanging onto a platform, etc. Work all the math above on each type of activity- relaxed/normal/stuck in a current while trying to bring a porthole to the surface....this gives you a much better benchmark.
As for adjusting weight, steel 72s are heavier, and you should need less weight.
Hope this helps, and if I missed anything in the formulas, sorry, its a bit early for me right now.
-Jay