An annoying oversight in Livesheets is the lack of inverse trig functions. They will be turning up in the new version, when it’s complete. But in the mean time, this sheet calculates the arctan of a number (in radians) using a polynomial expansion.
This gives you the option to choose how many terms you want to go to in your expansion (the more you use, the slower it will be). 11 or so seems to be enough for most values. For numbers close to 1 or -1, it seems to need more, perhaps going to 50 or so.