If there were a reason to want a pH of 7, for example, while you have 30 ppm of CO2 in the water, you would add sodium bicarbonate until the KH is 10 dKH with the CO2 in the water, and you would have a pH of 7, KH of 10 dKH. If the CO2 dissipates down to 10 ppm at night, the pH would rise to about 7.5.

But, there are few reasons to want a specific pH in the tank. KH is the more important parameter as far as the plants and fish are concerned.

Adding CO2 has no effect on KH.
Adding KH has no effect on CO2 either.

You add precisely the same amount at high or low KH.
The amount of CO2 at ambient conditions(equilibrium with air) for a glass of water at a KH or 20 degrees is exactly the same as a glass with 0.5 degrees.

Not sure if this helps, but hopefully it'll make more sense.

If you add a specific amount of CO2, you get a specific ppm of CO2 in the water.
Fairly obvious right?

Now if you change the KH, that will not change the rate of CO2 you are adding.

You can remove most all the KH and still have the same ppm of CO2, you just cannot measure it well using the KH/pH table is all.

Regards,
Tom Barr