That gives the right order of magnitude for ppm of CO2 in water. So, it can't be way off in theory, but it could be ignoring other factors that would alter the ppm. As long as we don't have unusual conditions, like very cold or very hot atmospheric air, very high or very low atmospheric pressure, etc. that should work. (I can't think of any other parameters that might affect it.)
"Since the beginning of human civilization up until about 200 years ago, our atmosphere contained about 275 parts per million of carbon dioxide. Parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere.
is a very thorough discussion of CO2 in the atmosphere and in water, but it will take me many days to understand much of it.