[Freesurfer] creating surface intermediate between white and pial: mri_surf2surf and mri_surf2vol

I am hoping to create a new surface that is half-way between the white and pial surfaces, then project it back to the volume. I have seen some examples of creating normals this way, but I am unclear as to how to just create the surface itself. The Wiki page is not too helpful, as it does not list the -sval switch in the following example:

mri_surf2surf --s subject --hemi lh --sval-nxyz pial --tval ./pial_nrml_srf_lh.mgz

mri_surf2vol --surfvalpial_nrml_lh.mgz --hemi lh --outvol pial_nrml_vol_lh.mgz --identity subject --template T1.mgz --projfac 0.5

Is there another version of the mri_surf2surf documentation that discusses the —sval switch? For mri_surf2vol, I do not understand the –projfac implementation and cannot find a complete description of it. I would like to use it do do a smooth interpolation between the pial and white surfaces (with normals as well), but it is not apparent to me that this is possible.

Thanks, -Jeff