CO2 is very hard to mesure from PH/KH. most PH test are not 0.1 accure, and KH tests 2.
The best way to know your CO2 levels, is to buy a CO2 drop checker with a 4dKH solution and put some bromotymol blue in there.
In a 4dKH solution, the PH in the drop checker will change depending on CO2 in the tank, and will give you an accurante color.
Blue = poor CO2
Green = good CO2
yellow green = high CO2
yellow = too much
red = (You should see most of your live animals floating at this point)
I use it at home and it's very accurate. From my experience, invest in a 4dKH solution (eBay), making it at home with baking soda is not so good, the solution is dropping a bit in dKH overtime and KH test kit are not so much accurate.