When you use a drop checker you are using the pH test kit. The color of the water in the drop checker indicates the pH of that water. Since green is the easiest color to pick out accurately and it coincides with 6.6 pH, 6.6 pH with 4 dKH water = 30 ppm of CO2. But, if you are .2 off on the pH when you see green water in the drop checker, that is still a 58% error in ppm of CO2. As bad as that sounds, using tank water pH and KH very often causes 200% or more error.