Hi Peter,
If the secondary has been
off-set in the vertical axis, then the mark is placed on that "off-set centre". If there is no vertical off-set to the secondary then the mark is placed dead centre of the secondary. You don't want more than one mark on the secondary. Any more and it starts to get confusing.
The secondary on my big dob I off-set in one axis only, that one being away from the focuser. I didn't bother with the vertical one, so the mark is placed dead centre for this secondary mirror. From your thread reply, this is the situation with your secondary too, so the mark you've done is all that is needed,
.
The off-set I had calculated for my dob when I was designing it, if I remember correctly, was to be some 3mm down towards the primary. If I HAD off-set in the vertical axis, then the mark I would have place on the secondary would have been 3mm
above its geometric centre as it is only the geometric centre that has been lowered,
BUT 4.24mm along the plane of the secondary taking into account that the mirror is at 45* to the focuser (remember Pythagoras from school?).
It is important to keep this in mind or you will not achieve true collimation as the mark on the secondary will not be in its correct place.
Alex.