The main advantage provided by the mirror on a Mint or Wally is not the ability to see the stylus. I can see the stylus just fine too, my myopia is well known!
The advantage of using a mirrored protractor is that it enables parallax, which allows you to:
1. move your head to center the printed/scribed alignment grid on the top surface above its reflection off the bottom surface, thus ensuring that you're sighting directly along the centerline of the grid; and
2. with the stylus on the null point, adjust the cantilever zenith angle such that the cantilever precisely occludes its own reflection, thus ensuring that you're sighting directly along the centerline of the cantilever whilst it's centered on the null point and parallel with the grid.
You can't equalize/null the differential between two images unless you have two images. On card stock, there's no way of verifying that you're sighting directly down the center of the alignment grid *or* the cantilever.
Imagine trying to focus a split-image rangefinder lens without using the split image. All you could go by is the apparent sharpness of the image. You'd probably get close but you couldn't match the precision of matching split images.
Perhaps you're not using your Wally to full advantage?
