There's a lot of small places to get tripped up behind the decimal
point.  For example, the generic AL80 isn't rated at 80 cubic feet, but
is {usually} 77.4 ft^3.

http://www.luxfercylinders.com/products/scuba/specifications/us_imperial.shtml

At .08071 lb/ft^3, that works out to ~6.25 lbs for a complete tank
drain....77.4 vs 80 is a quick ~3% error.

Interestingly, when looking at Luxfer's chart (above URL), for their
7.25" diameter 3000psi "AL80" (part#S080), they project that it will
have a net weight change of (4.40 + 1.40) = 5.8 lbs

Taking Luxfer's number and working it backwards, division by 0.08071
suggests that their tank volume change is 71.9 ft^3, which since it is
listed as 77.4 works out to be that they assume their "empty" rating is
actually at around 200psi remaining.

FWIW, my advice to the OP is that a toy submarine is going to be like a
real one when it comes to ballast tanks:  you'll want it to have some
excess capacity in both directions.  How much excess depends on the
overall size of the sub:  you'll want to be able to become negative in
order to dive without using the control fins and forward motion, and
when you surface, you probably want enough bouyancy to be able to have
the conning tower to freeboard above the water's surface.

And another potential source of good practical information would be to
find the hobbiests who do radio-controlled (R/C) model ship-to-ship
combat.  I have heard that there has been at least one example of
someone coming to the fight with a sub and wreaking havok :-)

-hh