That's a hell of a lot of drawing
on the mirror to find the center IMHO.
The simplest way to do this is put the mirror upside down, draw the perimeter, find the center of the circle with the method indicated in the tutorial
on the paper or even easier fold the paper template in 4, then drop the center spot on the mirror by having it stuck on the paper while matching the edge of the mirror. Catseye has made such a template. No brainer.