What really are we talking about? We get around dosing checks with the EI method. Then, if we experiment using fish gasping, algae and pearling as a point of reference in upping or lowering our co2, is this just not another form of EI dosing? The chart and drop checker get no closer than we ourselves can predict with a close eye on our fish and tanks, and small changes in bps. Probably a better estimate considering we dont have restrictive limitations (numbers) and nor do our planted tanks.
In pre-drop checker days, when I was having loads of algae problems, I was frustrated because when I increased the bubble rate of my CO2 my fish would definitely show they were very uncomfortable with it, so I would stop increasing it. The chart showed that I consistently had around 60-100 ppm of CO2 in the water. I knew I would kill off the fish at that rate.
After I figured out how to make a drop checker work for me I found that the discomfort my fish were showing when I increased the bubble rate was mostly my imagination. I was able to greatly increase the bubble rate to get the green color, and not lose any fish. My algae problems eased off considerably.
I think many of us, new to pressurized CO2, are nervous about gassing the fish, so are gun shy about increasing the bubble rate enough to even get close to an optimum amount of CO2 in the water. A drop checker gives you the courage to go ahead and raise the bubble rate. Once you get over that hurdle, you eventually get to see what excessive CO2 does to the fishes behavior, then you can recognize it without using the drop checker again. I see it as a learning tool, more than as a precision testing device, which it certainly isn't.