Re: Terraforming
Good question. I can't give you a good quantatative answer but I think, If I remember correctly that an earthlike atmosphere without constant upkeep would take a million years or so to return to present conditions.
The first putative CO2 atmosphere was sputtered (removed) partially by an intense solar wind of a younger sun which is no longer the case although still a factor to consider.
So, yes. I don't think it would be hard to shield 40% of the flux of UV we see at Earth with a scaled down version of our atmosphere. The problem is though, getting the atmosphere in place before hand.