pH/KH/CO2 formula?

["Craig McPherson" <craig at laceyonline_com>]

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 
page:

http://www.hallman.org/plant/booth1.html

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.