Welcome to the forum! If you have pH of 6.5 and KH of 2 then I read about 19 ppm CO2.
But, the pH/KH table isn't the best way to determine CO2, due to margin of error in testing for both as well as other things like driftwood that can also lower CO2. Many people use drop checkers, or you can do a de-gas test where you let a sample of tank water sit for a day or two, test that pH, then compare to pH when you are injecting CO2. Ideally you want at least a 1.0 point drop. Since you aerate at night, that would work the same as a de-gas test, and you want that 1.0 pH drop to take place before your lights come on so CO2 is fully available to plants when they start photosynthesizing. IYou'll have to inject CO2 to achieve 30 ppm.