The pH/kH/CO2 chart is based on the assumption that the ONLY buffers in the system are bicarbonate based and that the ONLY thing lowering pH is CO2. If you have driftwood or peat in the aquarium it will add acids that lower the pH. Also some substrates will lower the pH. Some people add pH lowering chemicals thinking that adding them will add CO2.
I know that the water in my planted tank has a lower ph than my other one. This is because of my driftwood and perhaps my flourite substrate, and my ph came down from almost 8 to 7.2 before I started my co2. So if I understand you any ph reading I get from my tank cannot be used with the co2 chart, correct? Can I use the ph reading of my tap water, since it is the water I put into my tank, for the co2 calculation?