Well, tap water is not going to tell you much. If it has a high KH then it will take tons of co2 to make it change color and just the opposite if it has a low KH. The color means nothing if you do not know the exact KH of the solution in the drop checker.
Also note that it can take several hours for the solution to change colors. A drop checker is really telling you what the co2 level was a while ago. For more instantaneous results you should use a PH pen or check PH with a test kit. Take a sample of tank water in a jar and aerate it for 1/2 an hour using an air stone. That drives all the co2 out and this tells you the true PH of your tank water. Then test the PH of the tank after the co2 has been running for a couple hours. You are looking for a difference of a full 1.0 in PH value. For example, my PH when de-gassed is 8.2 and when my co2 is running it is 7.1
Once you get to that point you can make slight adjustments very slowly until your fish show signs of stress and then decrease the amount slightly.