Most of the better scapers seem to eyeball cO2, they start with some relative measure of dosing for CO2, then they tweak and adjust slowly from there.
I do the same thing with the pH/Kh chart, but not those charts above
Also, as you depress the pH with CO2 gas, the concentration will increase a lot more(eg, it's non linear) for each 0.2 units of pH.
Say you have a KH of 3 degrees.
At a pH of 7.0 you would have 9 ppm
At a pH of 6.8 you would have 14.3 ppm
At a pH of 6.6 you would have 22.6 ppm
At a pH of 6.4 you would have 35.8 ppm
At a pH of 6.2 you would have 56.8 ppm
At a pH of 6.0 you would have 90 ppm
Differences between each 0.2 pH units:
So your pH measurement and observations need to be very good when you use more CO2. If you over do things at the higher ppm's, it only takes a little bit of change to dramatically increase the CO2.
This is one reason why many people fail when adding more CO2 and gas their fish instead. If each 0.2 pH units were only 5 ppm difference, then it would be pretty easy to adjust CO2. This is also a good reason to buy a nice CO2 regulator, needle valve etc.
Ah but what do I know, hehe
Since many use the drop checkers and there's little differences between the colors and those color changes are at best, 0.2 pH, what does this say at the higher ppm's of CO2? Not much.
Or if they use colormetric pH measure? Similar.
A good 0.01 accuracy pH meter is likely the best relative measure for CO2 using pH.
I knock my pH down about 1.4 pH units. This is about 47 ppm.
If it went to 1.6, then I'm at 75 ppm's, if I back off just a hair, 0.1 pH units, then I'm about 1.5 pH units, I'm at 59 ppm. 1.3 pH units, 38ppm, 1.2 pH units, about 30 ppm.
Tweaking CO2 is not some simple thing. It's not something to just wing it and assume the drop check has to be correct. You need to be careful.
A good pH meter can help make small tweaks and adjustments.