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pH/KH/CO2 formula?

We've all seen charts that let you find CO2 content by cross-
referencing KH and pH values, like the chart at the bottom of this 


Surely there is some formula that these charts are created with.  
Does anyone know what the formula is?  I'd like to know what it is 
so that I can plug in any values I want and solve for any variable I 
want rather than having to consult a chart with a limited set of 
values.  I can see trends on the chart (like a tenfold increase in 
CO2 means a full one-point pH drop), but that's not enough for me 
to figure out the formula involved.  I'm sure the formula would involve 
logarithms, but that's why I kept my TI-86 from high school.

Also, the above-linked page says that a normal equilibrium level for 
CO2 in water (no CO2 being produced or consumed, just 
equilibrium with the atmosphere) is about 2-3ppm.  Is this correct, 
and is there a more specific/exact value, or does it vary?

If you reply, please CC me; I'm subscribed to the digest and want 
to start playing with my brand-new CO2 cylinder soon.