Have you tried this method?
http://www.deepskyinstruments.com/tr...re_Ver_1.0.pdf
The problem with using the centre dot as reference is that the center of the mirror is not necessarily the center of the optical axis. The above method uses on and off-axis astigmatism to align the mirrors.
There is also
https://www.ccdware.com/products/ccd...tor/index.html which will give you a 3D image of your focal plane and help you adjust it.
Best,
Markus