Without a drop checker an inexperienced planted tank hobbyist can't really tell if he is putting 5 ppm of CO2 in the water or 20 ppm, and there is a huge difference in effect between those numbers. With a drop checker you can pretty well ignore the fish behavior as you slowly increase the bubble rate, until the drop checker color is green. Now you know you are somewhere near 30 ppm of CO2, which is a good goal. Then, the best idea is to increase the bubble rate a tiny bit, watch the plants and fish for a day, looking for the plants to pearl sometime in the middle of the day, and the fish not to cluster in a top corner "gasping" for air, or lay on the bottom, or other similar odd behavior. Do that slight increase once a day, until either you get the pearling or the fish do act distressed. If it is the fish, back off slightly, and leave the bubble rate there until something else changes in the tank.
The green color just gets you in the right ballpark, and may indicate 20 ppm or 45 ppm, or somewhere in between, depending on your ability to judge the color. And, never forget, the drop checker is only telling you what the average concentration where it is located is, but other spots in the tank will have much different concentrations.