The MG811 spec gives you the range of measurement, but without the values and connections of the surface mount op amp on the board the only way to determine the voltage output for each CO² concentration is with a Voltmeter and an already calibrated CO² PPM meter, which you could perhaps borrow from a lab or university. Once you have the CO² values and Voltages that correspond, you can build a formula or function to convert the digital values to voltages and then to CO² concentrations.

The other option is to trace the tracks on the PCB and create a schematic and then post the spec above and the schematic with resistor values included on the electronics Stack Exchange.

The MG811 spec gives you the range of measurement, but without the values and connections of the surface mount op amp on the board the only way to determine the voltage output for each CO² concentration is with a Voltmeter and an already calibrated CO² PPM meter, which you could perhaps borrow from a lab or university. Once you have the CO² values and Voltages that correspond, you can build a formula or function to convert the digital values to voltages and then to CO² concentrations.

The other option is to trace the tracks on the PCB and create a schematic and then post the spec above and the schematic with resistor values included on the electronics Stack Exchange.