The chart that shows ppm of CO2 vs KH and pH works fine only for water that contains nothing affecting pH and KH except carbonates (bicarbonates) and CO2. Water in an aquarium almost never meets that criteria, so the table almost always gives a ppm that is much higher than it really is. The only accurate CO2 measurement you can make, using KH and pH of the tank water is when you keep the KH constant and measure the change in pH from when you have no added CO2 in the water and when you have injected CO2 into the water. When you measure that difference you have added 10 to the change in pH power times the ppm in the water before you added CO2. It is believed that the ppm of CO2 from the atmospheric CO2 is about 3 ppm, so if you add enough CO2 to drop the pH by 1.0, you have 30 ppm of CO2. This method is also inaccurate when you use a test kit that requires you to match the color of the sample water to a chart of pH vs color, because it is extremely difficult to judge that color close enough to get any accuracy. (If you think you have dropped the pH from 7.0 to 6.0, you may really have dropped it from 7.1 to 5.9, or from 6.9 to 6.1. The first means a ppm of 48 and the second means a ppm of 19. That is a huge uncertainty!) And, the ppm of CO2 from the atmosphere can be at least as low as 2.5 or as high as 3.5, so that makes the uncertainty be as much as 16 to 55 ppm.