You can't measure how much CO2 is in the water by measuring pH and KH. That only works if your water contains nothing that affects the pH except carbonates and CO2. Very few of our tanks come close to that. The best way to "measure" the CO2 is to watch the plants and fish. Good CO2 will cause the plants to grow the fastest that the light will let them, sometimes causing pearling of most of the plants. Too much CO2 will cause the fish to show obvious distress, from clustering at the surface seemingly gasping for air, to losing their colors or laying on the substrate. With too little CO2 the plants will not be growing as fast or pearling as much as they should, given the amount of light they have. (This assumes that they are not limited in growth rate by a shortage of N,P,K or trace elements.)
A drop checker is a good way to find out if you are anywhere near the optimum amount of CO2 in the water. If used correctly, it will be green to yellow-green if you are at least in the ballpark on how much CO2 you have.