The equation that the CO2 vs KH and pH is based on the assumption that there is nothing in the water that affects the pH except carbonates and CO2. That isn't the case with our aquariums, so that chart doesn't work.
If you get a drop checker, fill it with 4 dKH distilled water (not tank or tap water), and add a couple of drops of bromothymol blue pH reagent, and put it in the tank, the water in the drop checker will be green to yellow green when the aquarium has about 30 ppm of CO2 in its water. After that you can slowly, very slowly, increase the CO2 bubble rate looking for good pearling of the plants and for signs that the fish are being bothered by the CO2. Beyond that, you need to spend about $2000 for the device that can accurately measure the concentration of CO2 in the tank water.