For a new tank, it just takes some time to see what it settles on once all the new stuff gets done. moving the numbers. And from there it can be kind of an "estimate"?
We say that 30 PPM is a good level for CO2 and that is then assumed to be done when the initial PH has dropped by a full point in PH. That is what the drop checker really does. We start out and the color is one point of reference and when it then turns green it is supposed that the PH is down a full point. But to get there we are kind of making a few assumptions along the way which may not totally be true so we may need to tweak things just a bit to get the best growth out of the plants.
If the only thing causing the reading of your KH is carbonates, the chart is a big help but then there are often other things besides the carbonates showing on the readings. No chemist here and just passing on things I read to let others explain it far better. Just don't get too involved with making changes to get exact numbers as much of what we measure is based on estimates. How exact is the estimate? We often don't really know so we estimate and call it good but then if we keep in mind that it IS an estimate, it will relive a lot of stress over trying to get exact numbers and eventually we get more into looking at what is happening with the plants and fish. Plants and fish doing well is more important than the numbers but all of this does take time and patient waiting to see what's really happening.
What to do now? Go with the "expected" and estimates. If your PH starts out at 7.8 and you have dropped it to 6.8, then assume the CO2 is correct. But if it seems like the plants are not doing well and pearling, etc. it is not a terrible decision to turn the adjustment down to get 6.6 as long as any fish are not showing signs of stress. Too much CO2 will not kill the plants nearly as much as any fish.
Being a gardener takes a good deal of patience. Being a gardener , inside and under water takes even more?