Dear Jon: I'm not familiar with the geometry of your Notthingham tonearm or its protractor.
If you can try to find the DB System protractor ( inexpensive one or a similar device ) and do the alignement with it. This protactor use the two null/points design, it is supposed that at these two points any pivot tonearm has cero traking error ( angle cero degrees ) and when the cartridge is perfected aligned in those points then everything is right on target. If your protactor works around these to point design and your cartridge is aligned with it then you have not to worry about " suqare " or anything elese.
Regards and enjoy the music.
Raul.
If you can try to find the DB System protractor ( inexpensive one or a similar device ) and do the alignement with it. This protactor use the two null/points design, it is supposed that at these two points any pivot tonearm has cero traking error ( angle cero degrees ) and when the cartridge is perfected aligned in those points then everything is right on target. If your protactor works around these to point design and your cartridge is aligned with it then you have not to worry about " suqare " or anything elese.
Regards and enjoy the music.
Raul.