Correct lens distortion with ffmpeg
In the manual it says:


  To use opencv use the calibration sample (under samples/cpp) from the
  opencv sources and extract the k1 and k2 coefficients from the
  resulting matrix.


If you go to the opencv website there's a sample source code for a lens correction program, written in c++, here. You'll have to compile it yourself, after installing OpenCV (sounds like you better put on a pot of coffee).
When you run it it will spit out an xml file containing the values for k1 and k2, which you can then use as inputs to the filter.