For the Geology part of planet generation show the atmosphere in actual mbars

[Fritz1776]

It doesn't vary during biosphere generation, and often times something like "earthlike" is meaningless since it could be well above or below the planet's gravity. Actually showing the number would make far more sense.

[zgrssd]

Atmosphere numbers can be changed by both the Biosphere and Colonisation steps. Mostly plants converting CO2 into O2.

And if stuff like Chlorine is still disolved in the atmosphere can be heavily dependant on the geology.

[Fritz1776]

Yes, but the total atmospheric density (mbar) is constant. It is clear that this is defined in the geology phase, so while biosphere determines the composition it doesn't determine the air pressure.

[zgrssd]

ORIGINAL: Fritz1776

Yes, but the total atmospheric density (mbar) is constant. It is clear that this is defined in the geology phase, so while biosphere determines the composition it doesn't determine the air pressure.

That is a faulty asumption. Turning CO2 into O2 or adding/removing certain stuff of course changes the desnsity - and thus airpressure - of the atmosphere!
Not big enough to be relevant on a human scale, but this is a planetary scale.