The problem frequently is people don't know how to mount the gun correctly. The gun is brought to the face not vice versa.
This is primarily a problem with "high gun" shooting, and "high gun" fitting of inexperienced shooters. It's hard to think that someone could shoot "low gun" and hit anything, while doing this.
And that's part of the problem with the method you specified. I have no real problem with testing where a gun points, but the precise formula for trimming the stock suggests that this test is far more meaningful than it really is.
If someone brings the gun to his/her face, and finds that he sees a little rib and a bead, straight in line with the eye, it's time to do some basic patterning, or even just "dust kicking", then start shooting.
Even with an experienced stock fitter working with an experienced shooter, it takes some trial-and-error work. Until someone can shoot
reasonably well with a gun that fits
reasonably well, it might be impossible to get a perfect fit, or even a near-perfect fit.
And a gun that doesn't fit
at all is pretty easy to ferret out just by having someone mount the gun and look down the barrel. Mirrors and other tools can help with determining basic fit, but familiarity with the shotgun is an unavoidable prerequisite to getting any farther with it.