Has anyone crunched the numbers on colonizing planets when starting the atmosphere from scratch? If not I'll look at the numbers when I'm home. But what I mean is, there's the minimum amount of oxygen needed for breathability, plus enough other gasses to make oxygen under 30%. So the boundaries of the range would be whatever temperature you'd get if you filled up the 70% with either greenhouse or anti-greenhouse gas. Any planet with a temperature within that boundary would be effectively the same amount of work to terraform, assuming there was no atmosphere on it. Or does the base temperature of the planet change that calculation significantly? Can't recall ATM.
Greenhouse factor is a multiplier for the base temperature, so it matters for those calculations. I have done some work on calculating terraforming atmospheres, but not exactly in the way that you mean.
If you want to build "garden worlds" that are as Earth-like as possible, this pseudo-code will tell you what mix of gases to use:
Oxygen = .2
If Base Temperature x Albedo < 95.6667 [too cold to reach 14C]
Safe Greenhouse Gas = 1.8
Else If Base Temperature x Albedo < 151.0526 [pressure > 1 at 14C]
Safe Greenhouse Gas = (287 / (Base Temperature x Albedo) - 1.02) / 1.1
Else If Base Temperature x Albedo < 260.9091 [pressure = 1 at 14C]
Safe Greenhouse Gas = 287 / (Base Temperature x Albedo) - 1.1
Nitrogen = .8 - Safe Greenhouse Gas
Else If Base Temperature x Albedo < 956.6667 [pressure = 1 at 14C]
Anti-Greenhouse Gas = -287 / (Base Temperature x Albedo) + 1.1
Nitrogen = .8 - Anti-Greenhouse Gas
Else [pressure > 1 at 14C]
Anti-Greenhouse Gas = (-287 / (Base Temperature x Albedo) + 1.02) / 0.9
But that's only useful for role-playing purposes, or perhaps if (like me) you've massively overbuilt your terraforming capacity to suck the atmosphere out of Venus. I have yet to develop the equivalent algorithm for minimally viable worlds - i. e. , atmospheres that require the least terraforming to reduce colony cost to zero.
But I have done it on a case-by-case basis for the inhabitable bodies of Sol:
Mercury: 0.1 oxygen, 0.63 AGG
Venus: 0.1 oxygen, 0.0402 SGG, 0.1932 filler
Luna: 0.1 oxygen, 0.1852 SGG, 0.0482 filler
Mars: 0.1 oxygen, 0.156 SGG, 0.0774 filler
Jovian moons: 0.1 oxygen, 0.9946 SGG
Titan: 0.1 oxygen, 1.8 SGG, 0.1 filler
So while Mercury is frequently ignored because the colony cost is so high, it's actually easier to terraform than anything in the outer solar system.
Is there anything about actual in-game effects from changing those numbers? I have a vague recollection that having a xenophobia under 50 means minorities won't cause unrest in your empire, which I would do just for the spam reduction.
I'd say Luna is the better bet but you definitely would have to run 8-hour cycles in order to take advantage of it. Bulking it out with spaceports would be worth it as well, because the loading / unloading is a much greater % of time spent when you're travelling to Luna rather than Mars.
I believe militancy and xenophobia also affect PPV demand and police effectiveness, and determination affects population surrender. I have not been able to find any indication that the other values have any effect.
I usually just build a single spaceport on Earth and none in the colonies. They can't be moved, so I'd have to move the construction factories/brigades and minerals to construct them in place, which seems like more trouble than it's worth. Even on Earth, where most trips originate, the expense doesn't seem to justify the diminishing returns.