The pH/KH chart is often used to get a reading of CO2 levels in the tank.
However, many folks go about this the wrong way. If you are estimating CO2 levels using only 1 pH reading - then chances are you are getting an incorrect estimation of CO2 levels.
This is an incorrect approach because many other factors can contribute to the tank's pH value: fertilizers, tannins & other chemical reactions. Matching a single point value of pH and KH to get the CO2 level assumes that KH & CO2 are the only factors that determine pH. However, this is not so - there are other elements that can cause a drop in pH besides CO2.
If you measure pH/KH when tank is at equilibrium when CO2 injection is off, CO2 levels in the tank will match equilibrium levels with atmosphere, with respect to gas laws - which will be around 2ppm. A tank without CO2 injection will not have elevated levels of CO2.
In the example below, if I think the pH of my tank (before CO2 injections starts) and I get a reading of pH of 7 and KH of 6, the table states that I have 18ppm of CO2, (red arrow). I know that it's a false value caused by other components in the tank that contributed to tank acidity as a tank without CO2 injection will always be near atmosphere equilibrium (where CO2 <3ppm or so). This is a false positive. If I take a pH reading later on while CO2 is on, I need to factor this in.
As a very general guide, if you have KH values between 1 - 10 dKH, aim for a 1 point relative pH dropfrom the point when CO2 injection is not yet turned on to the time after it has been turned on and CO2 has risen to a high, stable equilibrium point. This will put the tank's CO2 levels around 30ppm +. If you are careful, many tanks can aim for a drop of 1.2 pH instead. This method is simple but accurate if your pH tests are well calibrated. So for example if your tank starts with a pH of 7.2 without CO2 injection - you should aim for a pH of 6.2 during peak CO2 saturation.
Always be around to observe your livestock when you are tuning CO2. Fishes show signs of lethargy when CO2 rises to uncomfortable levels and at critical levels, they will be gasping at the water surface. Certain species will be more sensitive to CO2 levels (Discus for example) than others. That is why CO2 targeting must be relativedepending on observation of tank livestock and not just an absolute value. When releasing livestock into the tank, always release them during the period that CO2 injection is turned off. When levels build up gradually during the next CO2 cycle they will have some time to adapt.
It takes about a week for plants to fully re-program and optimize their enzymes to match available CO2 levels. The true gauge of what a good CO2 level is must be done by direct observation of plants across a couple of weeks. Stunted leaf tips, thin stems and new leaves that are smaller in size compared to older growth are all severe signs that CO2 levels are low (if light & nutrients levels are good).
Gauging CO2 levels by observing plants can be difficult for newer aquarists because it requires knowledge of what good plant growth form is for a particular plant. Comparing pictures to online examples is a start, but one must build up the observational experience over time to be really good at detecting whether CO2 levels are good.