Peter,
not sure (still) what you are asking.
Shoot a one second shot with the guide camera. I like to use a star about 5th mag or so, but that is me, using a Lodestar, and a 50mm finder, so yours may be different. Contrary to what others have said I like the star to be pretty crisp to start with, so eyeball it first. Again, different to some, I like the count to be about 40,000, and if it gets to 65,000 then I find another.
Then draw a box around the star you want. You may have to check "sub-frame" or similar so that the box gets used, it is different I believe on the latest V5. Having the smaller box allows quicker downloads, and speeds the process up. I usually use "Display Large Statistics" as well, in the "Options" section of "Expose".
Focus using the readout you get, looking for the highest number(s).
Make sense?
Gary
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