[Capt Jack Harkness]

I seem to remember SH4 letting you get to 100% before you die, even though it gives you the warning like SH5 at around 8%.

Realistically, "Concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour." -From Wikipedia.

Will do some testing to find the limit in game.

EDIT: The game will let you go all the way to 100% before you die. And of course, the crew warnings refer to oxygen, not CO2, i.e at 75% CO2 the crew report, "Oxygen down to 25%."

Perhaps the gauge could be set either to oxygen concentration with a scale starting at maybe 25% (atmospheric is about 21%) with a yellow zone between 20% and 15% (minimum O2 concentration for respiration at sea level is about 18%) and red below that.

Or you could leave the dial as is but change the scale so that 8% on the dial shows up at 100% in game...