That's a good question, but I don't have an answer. When I see the fish all gathered at the surface, I immediately look at my CO2 regulator to see what is happening. It is always the tank running out of CO2 - the tank pressure below 500 psi. Then I reduce the regulator setting to get an output pressure that is closer to 20 psi, and I start figuring out when I will visit my fire extinguisher service store for a refill. I never even think of looking at the drop checker then.
About a year ago I gassed (2) of my (4) discus. I was there to witness it and was able to bring them from the dead. Both discus were flat on the botom of the tank breathing every so slightly. I turned off the CO2 solenoid and started an airstone. The (2) discus came around in about an hour. Interestingly the (2) discus that were gassed were of the same variety while the other (2) discus that didn't have any problems were of another variety. At that time I did not have a DC but I do remember my pH meter showing a pH of 6.3.
So from now on, I get nervous once the pH gets down in the 6.3 range even though this pH may not gas the fish today.
I have some BBA in my tnak and I have been raising my CO2 a bit. I run about 1-1.5bps off a AM1000 with an Eheim 2217. My pH in the morning is about 6.9 and at the end of the day yesterday was about 6.4. My DC was reading a light green. All fish seemed okay. But I am getting nervous about it. Nervous enough where I want to be home to make sure everything is okay.
Something else I have noticed is my Cal Aqua DC is green most all day even before the CO2 comes on. It looks equal to the reference chamber color. The DC color doesn't change a whole by the end of the CO2 time period even though the pH has dropped by 0.5. Does that seem odd to you?
i think I would change that solution... a change of 0.5 should produce a visible color change in your DC if it's from co2 injection.
My DC has gotten as far as yellow with a tiny tinge of green. No fish stress evident. I have a lot of surface circulation though with a hob, plus an internal power filter which doubles as my co2 reactor that moves things around a bit. I can't remember now where I read it, but different fish have different co2 thresholds.
If it's not 4KH exactly, the results could be different too. For instance if it has a higher KH, it will stay blue/dark green longer, and a lower KH, will turn green/yellow much sooner. So you might want to check your reference solution too.
When I was at Tom's open house last month he mentioned that it appears that small fish can tolerate much more CO2 in the water than big fish. When you combine this with his discovery that the concentration of CO2 in the water varies widely across the tank, it probably isn't possible to define what the maximum concentration of CO2 can be. Perhaps the only role a drop checker can play is to get you near the area of "good" CO2, but you have to adjust the CO2 depending on water circulation, size and type of fish, and who knows what else to actually determine that you are as high as it is safe to go.
The amount of ferts needed is a function of light intensity, plant mass, types of plants involved and their growth rates, and possibly other parameters. The "estimative" part of EI may just have to remain a crude estimate. But, we do know that going well past the required amount doesn't do significant harm as long as we keep up with the weekly water changes.