Okay, please let me try and explain a couple things.
1. The pH/kH table as a way to measure your CO2 content, is at best, very imprecise, going in the direction of unreliabe. That table assumes you have water, some carbonates, co2, and nothing else, almost pure water. The table assumes that pH shifts are limited to CO2. 99% of the time, this is not the case. Changes to the pH by a bit of acidity completely throw the table off. I would have 100+ ppm of CO2, for example, which is just not true.
2. Forget about being precise measuring pH (e.g. calibrated pH probe) or kH (e.g. Lamotte tests), if in the end you resort to the pH/kH table, and end up comparing the results in pure water with your tank water.
You could rest your water for some hours, or use an air stone, to find out your water pH when CO2 is in balance with the atmosphere. That DOES give you the pH drop.
3. The problem is with people saying 1 point pH drop equals 30ppm. That might be the case, might not. George Booth and his studies came up with this value of "minimal" CO2, which varied from 2 to 3ppm in a tank. This is water in a tank, with active fish and bacteria. If we consider this as a starting point, a 1 point pH drop would add 20 to 30ppm.
What is the reasoning behind this? All gases will, given time, balance themselves between air/water, as explained by Henry's law (partial gas pressure, solubility, etc). So, given time, the roughly 400ppm of CO2 in the atmosphere will cause our water to have around 0.5ppm of CO2. Inside our homes this initial co2 value would go even higher (e.g. 1000ppm), which again would put more CO2 into the water.
Water, resting outside the tank, is getting its CO2 exclusively from the atmosphere, the air in our homes. In the tank the CO2 content also gets some help from all the fauna. Guessing its precise CO2 concentration is not easy, given the situation described. Consider that changes of temperature also change the situation, as warmer water will retain less CO2/gases in general.
4. CO2 injection, being very very short, shifts CO2 by 10x for every 1 point pH drop. So, if you have a 1 point pH drop, the initial CO2 concentration is now 10x higher (e.g. from 2.2 to 22ppm of CO2). That is regardless of kH, but you must have at least ~3 dkH to measure the pH drop, or pH crashes get very real.
Thats one way to measure CO2 content, by the pH drop. But the measure itself is the drop, not somehow a translation to CO2 ppms. Example: "I have a 1 point pH drop", "I was able to have a 1.4 point pH drop", or even "I gassed my shrimp to death with 2.0 points pH drop". With a drop from pH 8 to 6, the CO2 concentration is 100x higher (e.g. from 1.2ppm to 120ppm).
5. Most people have great results with 1 point pH drop. Some, because of several reasons, like the 1.4 to 1.5 region. That is up to you. My interest in this discussion is only clarifying that nailing down the exact CO2 value is not easy, but also not needed. Measuring the mililiters of CO2 being injected per minute is also another way to go, although in this case you have to consider dissolution.
6. Tom Barr once asked us "How many ways do you know to measure CO2?" He mentioned that he had 8+ ways available to him. Makes you wonder, right?
In the link below, he describes a very DIY/Precise way to check it, using a probe, O2/DO membranes, 4 dkH solution, and some effort. If you want to get precise, go for it
Originally Posted by Maryland Guppy
every 1.0 drop in pH = 30ppm CO2
7.2 - 6.0 = 1.2 x 30 = 36ppm/CO2
Im afraid this calculation is mistaken.
EVERY 1 point pH drop raises the pH concentration 10x, instead of adding a fixed amount. pH drop and CO2 concentration are not linear.
So, by the mistaken calculation, from pH 8 to 6, we have 2 x 30, so 60. That is not a correct calculation.
Keeping it simple, it is like a geometrical progression, and it needs a starting point, for instance, at pH 8 I have 2ppm of CO2, at pH 7 I will have 20ppm. That is why most people resort to George Booth and 2-3ppm of CO2, to have this initial value. Important to understand is the fact that if you drop your pH from 8 to 7 you have a added a smaller concentration of CO2 than if you lowered your pH from 7 to 6.
pH 8 - 1ppm CO2
pH 7 - 10ppm CO2
pH 6 - 100ppm CO2
That is exactly why people gas their fishes to death: just a 0.2 drop might raise the concentration a lot in a lower pH (e.g 6) while it adds almost nothing at higher pHs (e.g. 8).