This is the formula:

CO2 (in PPM) = 3 * KH * 10( 7-pH )

This is only accurate assuming that you have nothing in the water to change pH other than carbonates, which is why it doesn't work reliably in most practical situations since pH can be skewed by many things other than carbonates.

Injecting co2 reacts with water in this way:

H2O + CO2 = H2CO3 (carbonic acid) a weak acid

KH = HCO3 (bicarbonate) a weak base

Carbonic acid and bicarbonate do not react with each other....the bicarbonate works as a buffer by reacting with strong acids and creating a weaker acid so the pH doesn't drop as much....for example -

HCl (hydrochloric acid) + HCO3 = Cl- + H2CO3

Therefore water with enough carbonates in it will be able to handle a larger amount of additions of strong acids without causing the pH to drop as much as it would otherwise (hence the 'buffer'). But adding the weaker carbonic acid by injecting co2 does not react with the bicarbonates, and therefore the amount of bicarbonates in the water prior to adding co2 has no effect on the drop in pH due to adding co2, as they simply don't react with each other.

If any of this is inaccurate someone correct me....I'm not a chemistry expert by any means.