Jim Weir's CO detector design in June Kitplanes

In Jim Weir's CO detector design in June Kitplanes there is an adjustment
for tempurature and altitude. Can anyone please explain how to use the
adjustment in the circuit, ie, how do you set the sensor initially on the
ground and how do you then adjust it for altitude and temperature changes?

I'm sure some experimenting would answer the question, but I would like to
understand the 'designed' method of use.

Thanks in advance for any info.

Michael.