If I've looked at the chart right, pH 6.6 dKH 6.5 equals 49ppm CO2?
Doesn't a "drop checker" actually measure the pH (using bromothymol blue), not CO2. CO2 is then deduced from the chart according to your KH?
If your "drop checker" is green, using bromothymol blue, pH is ~6.6. Therefore, if your dKH is still 6.5 your CO2 is ~49ppm. However, if you have reduced your dKH to 4, then CO2 ~30ppm. (using the charts)
If you are interested, I am using excellent LaMotte CO2 titrator which could be used to verify your tanks parameters.