The optimum concentration of CO2 in the water depends on how much light you are using, and whether the plants you have are fast growers or not. When use of CO2 was still a novelty we needed to know what the ballpark number was for "good" CO2, so people, using the crude methods for calculating CO2 concentration, found that fish generally could live with 30 ppm, but might not live with more. So, that 30 ppm number was based on how the fish tolerated it, not what the plants needed.
If your lighting is around 30-40 PAR the plants will grow well with much less than 30 ppm of CO2, but if it is around 80-100 PAR, the plants will be stressed by having too little CO2, and algae will tend to take over the tank.
It isn't possible to accurately measure how much CO2 is in the water without spending a lot of money for an accurate CO2 probe. If you can be sure that your water has 3 ppm of CO2 in it from the atmospheric CO2, a drop in pH of 1.0 that results from adding more CO2 means you have 30 ppm of CO2. (That is 10 to the 1.0 power times the original pH.) But, atmospheric CO2 doesn't result in exactly 3 ppm of CO2 in the water. 3 ppm is just a reasonable approximation.
Also, using the pH/KH vs ppm of CO2 tables is not accurate unless the water you are testing contains nothing that affects KH or pH except carbonates and CO2. Typical aquarium water does contain other substances that affect both the KH and pH. Even if that isn't true for your water, it is very hard to accurately measure pH, unless you have a well calibrated pH probe/meter, and the calculated ppm of CO2 changes drastically with small errors in measured pH.
We just have to live with the crude nature of our CO2 ppm measurements, and accept that we don't know with any accuracy how much CO2 we have.