Having the mag set up being a mag in the machines taper I described in post 3, one might also want to have a center point, perhaps a good live center, a good 4 jaw chuck the fits the machine taper and a steady rest. Note thst all taper for devices should have a line-up mark to the spindle head so at removing and replacing they might still run close. It is not uncommon to grind the headstock center in place to rule out the head taper error.
I don't think the headstock mag is a very common method. It may not be suitable for large work.
Only a very good 4jaw chuck would be suitable because much grinder work is held to close tolerances.
Some ID and OD work is held with the but end held in a chuck or center, and the out end in a steady so to hold high accuracy and alignment of one end to the other. Much OD work is held between centers.
Good to have an indicator mounted to the table swing so to adjust the part straightness, you move the table half the error
based on the part length and how far you are from the table pivot point. Often one will hand feel the wheel to the part after measuring both ends to get .001or so close before setting on the indicator fo final adjustment to straight... the cross feed dial is used as an indicator to adjust the movement of the part.
The ability to make straight quickly is the art of fast grinding. One can waste a ton of time traveling end to end a number of times. One travel should get one to .0002 - .0005 if you think about the length of the part and how much is past the table pivot point.
Often you cant take a grind and measure because you may have little or no stock on the part. sometimes one may need to move the part's center back to zero before grinding that part. I used a Norton center abrasive point for this task.
Have a taper cleaning device to keep all grits out of the taper.
Norton 1 X 2 In. 1/2 Spindle A80-VVM Center Lap Mounted Point 80 Grit A/O, 61463622926 | Hillas.com