Depends. On the person and what is there to help.
Eyeballing is often good enough for many people, the item may not need perfection.
If you have another object that is in the right place you can set the 3D cursor to it. Or to any mid point of a loop of verts or some such selection.
With the 3D cursor positioned then you can move the origin/mirror point to it. This would be the most common and standard method.
General Q&A note
Help us all to help you.
Please give full screenshots with any questions. With the relevant panels open.
Also, include the lecture time and name/number that is relevant to the problem/issue.
The screenshot can be done by Blender itself, via the âWindowâ menu bar top left hand side.
On that menu drop down is âsave screenshotâ.
Close ups additionally where they help.