I've recently made a spreadsheet to help me with terraforming. It's far from being perfect, but I would like to share it with you. Basically it allows you to specify what temperature and gas proportion you want in a planet, and after filling in the planet's base temperature and albedo (which has to be changed if its ice sheet melts for updated and accurate results) it will tell you how many atm of what gases the planet needs in order to achieve your desired results.
For RP reasons I like to maintain Earth proportions in my terraformed planets. However, you can set Nitrogen and Argon to 0, leaving Oxygen to its minimum value of .1 atm and change your desired temperature to the minimum (or maximum) in Kelvin, which I think it is 273 and 317. The spreadsheet will automatically tell you if a safe gas is needed to reduce Oxygen bellow 30%. There's also a limit in how much you can cool or heat a planet. When it's impossible to achieve your desired temperature the spreadsheet will stop adding (anti)greenhouse gas when reaching that limit.
I'm not used to work with spreadsheets and I admit this one is pretty limited, but if someone find it usefull It would be great. As I don't know if the attachment will work for xls files, I will add a fileserve option.
http://dl.dropbox.com/u/24883108/Terraforming.xlsIf you find errors I will try to fix them
Edit: Changed host!