Doug,
I know there was a large variation in that cartridge dimension in the "good old days" but I was under the impression this had more or less standardised on 9.5mm. So aligning the pivot to that dimension on the assumption it will be within a mm or two must introduce less error by almost a factor of 10 when rotating the cartridge than if the pivot is 9.5mm away.
A facility for an adjustable pivot to stylus distance together with a cartridge mount that only rotates is another way to do it to avoid the two settings interacting. But then an arc protractor couldn't be used. It would have to be one based on the Dennesen principle or a standard two point.
John
.