3: Add greenhouse or anti-greenhouse until temp is within tolerance
5a: add inert gas if total is below 0.3
5b: remove inert gases until colcost is 0
These steps are where you either have to do math, or just start guessing and hoping you don't miss the event as it cruises by - or worse, that you didn't guess too high and *then* miss the event as it cruises by, accidentally bjorking your colony cost.
And why do either when code can do the calculations such that the atmosphere comes out perfect every time? By adding a button to use for automatic terraforming for populations you don't take anything away from the game - you can still terraform just as before. You're still free to make all the wonky atmospheres you want, or to terraform for a population without using the auto-button. But for those of us that don't want to be assed with math or guessing + paying close attention for the terraforming events, it'd be a godsend that'd mean we don't have to pay any cognitive switching penalties and can continue focusing on fleets, resources, or whatever else we find more interesting. This sort of optional automation of features is a good thing, as demonstrated in almost every other similar game.
There are only two drawbacks that I can currently see to having a "Terraform world for <population>" option:
1) Steve has to code it (this is the big one, his workload being what it already is.)
2) It makes a very small but very tedious/unfun part of the game easier and less cumbersome for people, and I'll miss knowing that others are suffering from it because I hate people. (Please forgive the sardonicism. It's genetic.)