Using mris_find_flat_regions in freesurfer

Summary of what happened:

My goal is to use mris_find_flat_regions to find a small, flat patch of the cortical surface which I can export to process. I used the command on a lh.pial file, and it returned a wfile of relevant vertices.
The problem is I can’t work out how to use the wfile in a meaningful way; I have also converted it to an ASCII file, but this is as far as I have reached. I need to work out how to cluster the vertices together, then how to export the set of vertices pertaining to one particular cluster/patch of the surface.
N.B I’m using the cortical surface of the good_output subject in the Freesurfer tutorial data set so I know the data is good.

Command used (and if a helper script was used, a link to the helper script or the command generated):

mris_find_flat_regions lh.pial lh.pial.flat


Freesurfer 7.3.2

Environment (Docker, Singularity, custom installation):