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CO2/KH/pH Questions

I was looking through Amano's books last night and thought of the KH/pH/CO2
relationship.  Amano often lists all three parameters for each tank. 
Typical values are pH 6.8 and 2 dKH.  CO2 levels for such tanks are listed
at 15 mg./l or greater.

Yet, when I go to the KH/pH/CO2 chart I find lower CO2 values, sometimes
quite a bit lower.  In the example above 9.4 mg./l. is what the chart shows
(TOA pg. 182).  Has Amano missed the mark, or is the chart not "absolute,"
so to speak?

Second related question:

In the section on carbon dioxide, The Optimum Aquarium (English
translation) states:  "The greater the amount of bicarbonate dissolved in
water (or higher the carbonate hardness) the more free carbon dioxide is
necessary (see table)."  Section, pg. 79.  At 15 dKH the table
shows 50 mg./l of CO2.  Necessary for what?  The next sentence talks about
calcium precipitating out of the water if free CO2 falls.  Will calcium
precipitate at 15 dKH if free CO2 falls below 50 mg./l?  How does carbonic
acid fit into the equation?

Finally, I noticed that the optimum CO2 levels in TOA (green portions, pg
182) rise as the KH rises.  At 1 dKH 5-12 mg./l. CO2 is recommended, while
at 10 dKH 12-30 mg./l CO2 is recommended?  Does anyone know why?

Regards, Steve Dixon