So I did some more calculating and found this: They all seem to work on a simple calculation of the amount of ions that are added. Mkade, I have been thinking about this as well, but none of these calculators seem to use the pH for determining the equilibrium of carbonate and bicarbonate.
If you really want to convince yourself, you can take the pKa's of the three acid base equilbrium, and calculate the residual alklanity based on neutral water. What this means is that even if you fully dissolve 1g of CaCO3, you will not get the expected 1000 alkalinity, though you WILL achieve the expected 400 ppm of calcium if added 1g is added to a liter of water. At the same time, you're going to be losing some of the carbonate to CO2. Thus, as the pH is dropped, more CaCO3 can dissolve as the equilibrium is pushed towards bicarbonate and carbonic acid. If the pressure of CO2 in the surroundings is high, it won't escape from the solution (though this is not a factor in practical brewing practice). There is then an equilibrium between CO2 in solution and CO2 in the atmosphere.
From here, the bicarbonate can be protonated and turned into carbonic acid, which will quickly dissociate into water and CO2. So the second equilbrium, assuming a moderate pH, will lean almost completely to the right, so you'll really have bicarbonate in solution, and not as much carbonate. This is because of the following equilibria: For example, the 490 figure that you got from the spreadsheet will actually depend upon the pH of the water, and the pressure of CO2 in the surrounding atmosphere. It's been a little while since I was a TA for gen chem, but basically this is because of the various equilibria involving carbonate.