Normal people usually set up PS first and then align a cartridge in the headshell slots using chosen cartridge alignment method (Baerwald, Lofgren, Stevenson or whatever it is).
Some people do that other way around?
If your PS is wrong and tonearm already screwed to the mounting hole (and you can’t move it), then your chance to reach desired null points by moving your cartridge into headshell slots forward or backward is very little, sometimes it’s simply impossible! Or it’s possible just with 1 aligning methods of many (and probably not the one you wish to use).
Headshell lots are different size, depends on the headshell model.
Some headshells have fixed screw holes (you can't twist a cartridge in such shell changing alignment method).
Tonearms without removable headshell will give you even less possibilities, sometimes the slots are very small.
So the BEST method is to start from correct PS (always) !
Some people do that other way around?
If your PS is wrong and tonearm already screwed to the mounting hole (and you can’t move it), then your chance to reach desired null points by moving your cartridge into headshell slots forward or backward is very little, sometimes it’s simply impossible! Or it’s possible just with 1 aligning methods of many (and probably not the one you wish to use).
Headshell lots are different size, depends on the headshell model.
Some headshells have fixed screw holes (you can't twist a cartridge in such shell changing alignment method).
Tonearms without removable headshell will give you even less possibilities, sometimes the slots are very small.
So the BEST method is to start from correct PS (always) !