Our tanks do not have perfect water in them - they have other sources of alkalinity and acidity besides CO2 and carbonates. So, the pH/KH/CO2 chart or equation isn't reliable. If you use one of the ADA style "drop checkers" ( see DIY Drop Checker - Aquatic Plant Central- aquascaping...a living art) for more information, you can measure the CO2 in the tank with some accuracy. These little devices work because you can use distilled water, with a bit of sodium bicarbonate in it to get a 4 dKH sample of water that is "perfect" for calculating ppm of CO2 using a chart or calculator, such as Measuring CO2 levels in a Planted Tank. Quite a few people have now started using these devices and, so far, no one has refuted the claim that they really do work well.