Exactly, your background image is FSaverage, but Darren's tool is computing the vertices for the meshes he provides in
 https://github.com/dprice80/mni2fs/tree/master/surf
If you use one of his images, for example rh.pial.surf.gii instead of FSaverage, it will work. Darren's template has a different number of vertices than FSaverage. Darren's code is open source, so feel free to modify it for your desired template.