I don't really have enough time to give you an in depth answer or give it enough thought to give you a good equation, but I believe that some gasses work well according to the ideal gas law (pV=nRT) and some don't. This means some work more linear than others and you may have emptied half of the cylinder (cubic feet) before you get to half of the original pressure. I think CO2 is stored as a liquid (incompressible and therefore no volume change for pressure change), so throw that into the mix, and you get each gas is unique.

Given that, shielding gas is usually metered out in cubic feet per minute, which IS a constant rate so long as you have enough pressure inside of the tank to push it out. This means that no matter what, the tank is emptied at that rate, which is linear, no matter the pressure.

HTH,
G. L.

If I remember my physics correctly the noble gasses (like argon) are very close matches to the ideal gas. So argon or helium should be close to linear. The gasses which form diatomic molecules like O2 are less close but for these purposes probably close enough.
Bill

Another point to bear in mind.
If you are using a single stage regulator, as the residual pressure in the cylinder drops the pressure on the regulator diaphragm decreases which allows more gas to pass through the regulator so you need to keep re-checking your flow rates if it is for a critical application.

Cheers - Bob

Maybe you can just put an old bathroom scale under your tank and watch the weight from full to empty. :)