Fidelity Research FR64s Headshell dilemma

bukanona, What do you want to measure, tracking angle error or the distortion that results?  The former is completely predictable using geometry handed to us by the Greeks and their mathematical descendants.  The latter is complex, because you would need a test LP that encoded a single test tone, e.g., 1 kHz, from outer to inner grooves.  It would be useful to have such a test LP, but I don't think it exists.

@karl_desch 
You have read, experienced and understood the same problem. The only thing you mis-understood was that i had thought 230mm only allows stevenson.
I was perhaps under the mis-apprehension that 230mm ONLY allows stevenson.
@rauliruegas I appeciate you sending me the link repeatedly - but my question related to cartridge shell alignment using Lofgren A at 231.5 - not what the actual distortion figures are. However, it, is nevertheless particularly useful as it shows and confirms what @dover has said and pointed out - namely that at 230mm one can do Bearwald, Lofgren etc.
Likewise @dover having experiences the same head-shell length thing confirms i am not going insane - i ought to try a cartridge that is longer or has a longer cantilever to try out 231.5 or get a longer head-shell. Until then I will use 230mm
For my purposes thanks and thanks again for the advice.

Yes lewm measurement is a key. In the modelling then geometry was used to measure land in ancient Egypt results was easier predictable. Tonearm is much more complex.
To order such LP with 1 kHz tone is not big problem these days and even it's not expensive. Just someone has to make  methodology and try it for the sake of humanity :) After measurements we can discuss if 1 kHz tone is the right one but it will be figures not modelling. 

  

@nandric please read and look at the null points printed on the same protractor for different alignments. When we alight a cartridge we check 3 points step by step on Feickert. We do NOT altering PS, instead we just adjust cartridge position in the slots on the headshell to reach null points correctly. 

FROM DR.FEICKERT's PROTRACTOR MANUAL: 

"The general geometrical correlation of a tonearm based on a fixed pivot point are well known since the fundamental publications of Baerwald and Lofgren back in the first half of the 20th century. The conclusion of both papers was that such a tracking device must have an angular offset (zenith) with a defined overhang. The offset and the overhang vary with the effective length of the tonearm itself. While tracking the record the stylus matches tangential position in the groove at two points – the inner and outer Null points (linear tracking position). In the early 1960s another mathematician has calculated a new set of parameters taking into account that on records with classical music very often crescendos occur towards the innermost grooves (Stevenson)."

" **Step-1: Choose the geometry you want to use: Baerwald (B), Lofgren (L) or Stevenson (S).

Screw the gauge tower on the disc and place the Protractor on the platter. Aim with the pin in the gauge exactly over the pivot point of the tonearm. Move cartridge in the headshell so that stylus lands on the crosslines of your chosen geometry (B, L or S) at step 1 (overhang). In case you cannot reach either point on the Protractor please check your pivot-spindle distance.

Carefully tighten one screw a little bit and make sure you can still turn the cartridge in the headshell. Set antiskating to zero.

**Step-2: Turn Protractor so that you are over area of step outer Null-point). Set angular offset (zenith) so that the printed line on the Protractor and the cantilever of your cartridge fall in line when looking from the front. In case this is done carefully tighten both screws in an alternating way and take care the cartridge doesn‘t move anymore in this process. Caution! Don‘t overdo the tightening of the screws as really tight is followed by infinitely loose forever …

Double check your setup either at step 3 (inner Null-point) where the cantilever MUST fall in line with the printed line or you can alternatively check the overhang again at step 1. Both approaches are mathematical equivalent; if two points are correct, the third is correct by default. In case the cartridge still is off, then start again from the beginning by checking the overhang first and subsequently all the following steps again."  

" Having revised the math we found that for a given geometry all pivoted tonearms intersect with their tracking arcs in one particular point. This is a “unified overhang point” that can be used to adjust for overhang. Not only is the accuracy enhanced with this new Protractor, the complete setup process is much faster than it was before. "

Bukanona, I am still not quite sure I understand your point.  Surely you would agree that Euclid, Pythagorus, and the rest have left us enough formulae to deal with any nuance of tonearm alignment.  If you consult the original papers by Baerwald and/or Lofgren, I am sure you would see they relied upon the geometry we learned in high school, albeit maybe not every high school student could solve their problem.  As to the LP with a 1 kHz test tone, are you saying that I can buy such a test LP or that I could have one made?  I chose the frequency of 1 kHz merely as an example of a pure tone that would be a reasonable place to start to measure distortion induced by tracking angle error; it is not a matter of right or wrong.  Cartridge specifications usually list stereo separation at 1 kHz and cartridge output is often measured at 1 kHz. In the event, one might want to make such measurements at 100 Hz, 1kHz, and 10 kHz, and any other frequency of interest in between, I suppose.  It's a matter of how much time and energy you would want to put into it.  It would also be good to interface a computer between the output of the cartridge and the distortion analyzer, so that one could obtain a plot of distortion vs time and translate that into distortion vs location on the surface of the test LP.