Why do tonearm specifications list null points?

This happens all the time: we conflate tracking angle error with "distortion" without actual evidence that the two are perfectly correlated. And what sort of distortion might be correlated with TAE? Suffice to say it seems to be a good idea to minimize TAE, but whether that also minimizes "distortion" in a predictable or audible way is not established. Or if it is, where are the data?

You need a mirrored surface with lines finely etched and magnification.  

and certainty that the diamond tracing faces (assuming advanced profile) are aligned precisely 90° to the cantilever.

dave

Regarding a mirrored surface, I'm currently using the Dr. Feickert generation 2 protractor; while it doesn't have a mirrored surface, it has white etchings on a black matte aluminum surface are very precise and extremely easy to read (for me personally, this is easier to see than white etchings on a mirrored surface on protractors used in the past). I also have a very high intensity work light that I use from different angles (same height as stylus and shooting from the side or front as required) as well as magnification to ensure the diamond tip is in the cross-hairs. I'll admit I think I do have it easier than most because the Clearaudio Jubilee MC cart I'm using has the long cantilever extended out in front of the cartridge body making it very easy to confirm the cantilever's parallelity with the lines on the protractor.

With all this said, I'll be the first to admit that the improvement I think I'm hearing from switching back to to Lofgren A (Baerwald) from Lofgren B is a) either in my head because Clearaudio specified the Baerwald null points in their tonearm specs, or b) as @rauliruegas pointed out, most likely from tiny errors from my hand or sighting with the protractor.

Thanks all for your comments - always fun to discuss these things and learn from each other.  -cheers!