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Legault talks about a 2 step process. Rough collimation can be done at a lower power and patterns similar to that one will help you see that the mirrors are collimated. You do this just inside or just outside focus, but closer to the focus point than you've shown.
The next step is to examine a star's airy disk and diffraction rings at the exact focus point. This requires lots of magnification and excellent seeing. The larger the telescope, the smaller the airy disk, as larger diameter mirrors have increased resolution (they can discern finer detail). It's easy to see the airy disk and diffraction ring in a 4" refractor at 250x. A 12" reflector would require 600x or more and this an unrealistic magnification, almost never supported by the seeing conditions. Uneven cooling and tube currents etc. also complicate the matter with large scopes.
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