In that case, you'd have a regulator for your co2 tank (which is pretty much a requirement to even use a co2 tank, otherwise you'd just lose all your co2 in an instant), then you'd use a chart like this:
http://www.homebrew.com/pdfs/CO2chart.pdf locate the temperature of the liquid, and locate the requested volumes of co2, then you find what the psi would be, and you set your regulator to that desired amount.
Usually, for carbonated water/soda, you'd set your regulator to 30psi, then your water&gas mixture will result in 30 psi, which is... I can't recall the formula, but somewhere around 5 volumes of co2? (I just googled a volumes of co2 calculator for
http://www.brewheads.com/forcecarb.php, and their calculation, in reverse... if I wanted 4.7 volumes of co2 in my liquid, then at a temperature of 32 degrees, I'd set my regulator to 30 psi. Or if I wanted 3 volumes of co2 in my liquid, I'd set my regulator at 14 psi)