Grove Air Quality Sensor v1.1

I have acquired one of these sensors and I’m trying to give a meaningful interpretation to the output it provides.

The spec sheet of this sensor is not very clear on how it was calibrated. There is mention of a 0.25V ~ 0.6V variation in 20 ppm of CO, but this is before amplification. Given a 5V reference, I assume the range is translated to 0V ~ 4V as the sensor reads just over 4V (about 4.3V) when starting cold. Was the sensor calibrated to 20 ppm such that 4V = 20 ppm CO or is this wrong?

In other words, is there a way to roughly approximate the volume of CO in ppm from the value supplied by the sensor?
Some of the examples on the wiki page seem to provide a qualitative approach, while others normalize the input value to be in [0, 1) and then multiply that by 25 to return a result in ppm (which is essentially equivalent to what I mentioned above). Any help would be appreciated.