Since there are no outside variables in the known KH water, the CO2 formula is accurate in determining the exact ppm of co2 at any given time no matter what it's at equilibrium with (within a small margin of error).

The only thing that would matter would be that you can accurately measure pH in tank and known KH water samples, and that they come to equilibrium with each other. Whether they come to 3 or 5 or 10ppm doesn't really matter as long as they are the same. As long as they are both subject to identical conditions for a long enough period of time...I can't really think of any reasons why two samples of water sitting next to each other in the same type of container exposed to the same atmospheric conditions under nearly identical conditions would not reach equilibrium with each other. Isn't this the whole premise of the drop checker?

The question would then be how long does it take. I would think that since it takes a couple of hours or up to 4 hours for a drop checker to reach equilibrium with the tank water, it would take about the same amount of time going the other way, for tank water and known KH water to come to equilibrium with each other assuming you use the same quantities of each as you would use in your drop checker. Initial testing could be done just to see exactly when the pH stops changing or is changing in both solutions at the same rate which would indicate equilibrium.

Obviously if someone has a drop checker this wouldn't usually be needed, but I'm just saying as an alternative method it would be as accurate as a drop checker, or actually even more accurate if you use a pH probe, with a little more hassle involved. With use of a pH probe it should be possible to determine with very close accuracy the exact ppm of co2 you started with. With drop tests the accuracy would depend on how close you can read the test. If someone was concerned about whether the solutions actually come to equlibrium with each other, you could use two or three samples of each, the known KH and the tank water and test them all to see if there are a range of values or not. In a sense, you would be building a type of reverse drop checker, if you know the exact ppm of where you ended, and you know the difference in pH between where you started and ended, you know the co2 you started with.

No equation gives an exactly correct answer. The accuracy always depends on how accurately you make the measurements involved. And, because the calculated ppm of CO2 in water is proportional to 1 divided by 10 raised to the pH power, a small error in pH means a big error in ppm of CO2. That's why the drop checker, using visual judgment of the color, can never be more accurate (at 30 ppm)than 25 to 40 ppm. Using a calibrated pH probe. that is accurate to .1 pH, instead of visually judging the color, is a lot more accurate, but it still only gives a 10% accuracy. And, all of this assumes you know the KH very accurately. But, our test kits for KH are only good to about .25 dKH at best, so that adds another 6% or more error.

Fortunately, we don't need to know the exact ppm of CO2 in the tank. Knowing it is more than 20 and less than 40 is good enough for what we need it for. Less than perfect water circulation may give a bigger inaccuracy at any given point in the tank.

Yes, the obvious limitation for any of this is the lack of precision testing equipment. If someone wanted to spend a lot of time and effort they could probably get within 5ppm for sure (testing multiple samples for pH to eliminate error that way, and watering down the KH test kit to account for a 2.5ppm change in KH or something small like that). It isn't really useful to anyone to do this unless they don't have a drop checker, in which case it is at least a second option that will provide some measure of accuracy at least to the degree that we need to have.

As far as circulation, this seems to be a hot topic but with a lot of unanswered questions out there. It is always presented as a possibility when things go wrong, but at least for myself, I've never seen any evidence that a lack of co2 due to poor circulation is a factor large enough to be reckoned with in most normal circumstances. Wouldn't this be easily tested by using a pH probe to determine if the pH is different from one portion of water to another, thus showing a variability in co2 readings in those spots? If the changes in co2 concentrations are significant enough to be affecting the tank, wouldn't it be easily measurable? Has someone just not done this? Or are there other factors I'm not thinking about?