you might be surprise how much more air you suck at 80+ feet.
Get a good fill 3200 lbs in an 80 cubic foot tank. Quit at 1500 psi
and take your time on the way up. Two dollars of air ain’t worth your life.
You can play it closer after a few dives.
In North Atlantic diving we require doubles or a pony bottle. Just about every year we
get an out of air dead diver. A pony rig or SPARE AIR is a good investment.