CO2 (ppm) = 3 * KH (as CaCO3) * 10^( 7-pH) - This is the formula used that correlates to the chart on thekrib.com. It assumes that the only buffering system used is carbonate (CO3). Use of phosphate buffers or peat will negate any correlation to the formula. With your values you would need to have 9 ppm of CO2 in the tank.
This chart is most commonly used by Planted Tank People. In an ideal planted tank there should be 15-30 ppm for good plant growth. This is why most Planted tank folks inject CO2 into the water column. In my tank I keep a kH = 4° and inject CO2 to get the ph down to 6.6 (= 30ppm CO2)