http://aurora2.pentarch.org/index.php?topic=8495.msg101987#msg101987Taking the default humans as an example (as in, no tolerances have been altered when generating the game) this is how breathable atmo works. They have an ideal Oxygen pressure of 0.2, with a max deviation of 0.1. So, any amount of Oxygen from 0.1 and 0.3 is needed. To be clear, that number is atmospheric
pressure, not % of total atmosphere. You also need other gasses to lower the % that is Oxygen to 30% or lower. That 30% is an absolute set by the game, and has nothing to do with max racial tolerance. All gasses (including dangerous gasses and water vapor) work for this, but inert gasses are preferable: dangerous gasses quickly cause a CC cost at least as high as not having a breathable atmo, and water vapor will fluctuate until some time after you stop terraforming (assuming a temp range for both condensation and evaporation).
Also note, Aestusium and Frigusium are inert, meaning you can adjust for temperature and work towards getting Oxygen at/under 30% at the same time.
Of the dangerous gasses, Carbon Dioxide has the highest safe level, but even that counts as dangerous at 0.5%.