I don't think it really matters. The equilibrium is between the CO2 in solution and the CO2 pressure in the headspace. It's a property of the CO2, not really the liquid (beer and soda are both mostly water, after all). The only problem with that chart is that it doesn't go high enough if you want a highly carbonated soda -- the highest pressure listed is 30 psi. But you can at least get a sense of the relationship between temperature, pressure, and dissolved CO2.