[Freesurfer] smoothing

Hi, In the --help output of mri_surf2surf, the following explanation is given for --fwhm-src fwhmsrc:

Smooth the source or target with a gaussian with the given fwhm (mm). This is actually an approximation done using iterative nearest neighbor smoothing. The number of iterations is computed based on the white surface. This method is similar to heat kernel smoothing. This will give the same results as --nsmooth-{in,out}, but automatically computes the the number of iterations based on the desired fwhm.

What is the equation used to compute the number of iterations, and is it valid for an average surface?