That pH computer is actually a pH controller. It turns on a solenoid valve to inject CO2 into the tank to lower the pH to whatever pH value you set it for. There have been many discussions of pH controllers here, so there is no reason to repeat them.
There are many problems when trying to determine how much CO2 is dissolved into the water. First, the concentration of CO2 in the water varies widely from spot to spot in the tank. It can be very low in a densely planted area, while it is very high near the point where the CO2 enters the tank. And, it depends entirely on how good the water circulation is in the tank. Next, the pH/KH/CO2 ppm relationship depends on having water with nothing in that affects pH other than carbonates and CO2, but our tank water almost never is that "pure". At best you can watch the plants and fish, while having good water circulation in the tank and good water surface ripple, while slowly adjusting the CO2 bubble rate a little bit each day or two, until the plants are growing well, and the fish are not distressed. Then set the pH controller to whatever the tank pH is at that time. The pH controller then protects you from accidentally gassing the fish.
