So I actually made a patreon exclusive tutorial demonstrating how to use this .blend (for those with that perk) but it's pretty straight-forward:
1) load in your image in the middle plane (making sure it's relatively centered on the template - if not, scale move and rotate the plane until they roughly align)
2) use the smudge tool in texture paint to warp your face to the template (this will update the difference map on the right)
3) hit render
4) go to compositing and use the RGB node (the eyedropper) to sample the average color (the whole render should be one color after exiting the Levels node)
5) your score is the value in the luminance channel of the RGB node! mine is: 0.004413
see if you can get lower ;)