One drop checker with 4dKH can read anywhere from 18.9 and 47.5ppm CO2 depending on which shade of green you're able to discern.
Some indicators of low levels of CO2 would be GSA and stunted or slower then normal plant growth. Garry may know some more.
The main thing to watch are your fish.
One drop checker is good, but two are better. (or, wet killed a bunch of fish.)