don't know if you found the info you are looking for, this is from something i read a long time ago and maybe it will help you.

A "full" tank contains about 34% liquid CO2. If it is filled any more, the CO2 will become very sensitive to temperature changes, with a small increase in temperature causing a large increase in pressure. This is a dangerous situation which is avoided by only partially filling the CO2 bottle.

One cubic inch of water weighs 0.577 oz and the specific gravity of liquid CO2 is 1.977 gm/cc so one ounce of liquid CO2 has a volume of 0.877 cubic inches. CO2 bottles generally have a full-fill to volume ratio of about 2.57 cubic inches per ounce of CO2, so that one ounce of CO2 will take up 0.877/2.57 = 34% of the total volume of the bottle.

The figure of 68% is often quoted as the volume of liquid in a full bottle, but this error probably stems from translating "ounce" into volume using water as the standard. Water is 1.00 gm/cc, or about half the density of liquid CO2 so that if a CO2 bottle is filled to its rated capacity with water, it will be 68% liquid by volume.

you can read the entire article here if interested.

http://www.warpig.com/paintball/technic ... cs.shtml#3
also the only way you can know exactly how much co2 is in a tank, is to weigh the tank when completely empty and then again when full.

this may also help you out.

http://www.doomlabs.com/science/CO2_Science.html