Mounting hole-to-stylus variation / tolerance

Hi all,
I know this particular point has been touched on, raised, in some of the threads dealing with tone-arm-alignment in particular.
I truly think it deservers a dedicated thread since it has a more or less profound influence on various arm designs and their alignment.
It also has a profound influence in how accurate and beneficial any after market alignment tool will work for you!

We have had this far, feed back from Jonathan Carr, i.e. the variations and tolerances with regard to his product (Lyra).
We also had some rather powerful insights by Frank ..., who designs and makes arms i.e. the variations of various products he took note of.

Jonathan quoted, that his current products have a +/- 0.3mm tolerance from the 'standard' IEC...? of 3/8" or 9.52mm.

The findings of some contributors are different, possibly due to simple variations much greater than 0.3mm (Lyra's tolerance) or possibly due to subsequent movement of the 'motor' relative to the body, caused by rough handling, re-tipping, etc.

It is of most importance to get more feedback on this, due to the dependence of a close match to 9.52mm, on arms such as ALL of SME, to name but one. These rely on this to be fairly accurately aligned. If not, some pretty unacceptable misalignment follows as a result, if the stock alignment method is used.

Lastly, HOW can a 'normal' end-user measure this distance with some degree of accuracy?
I have no problem using a vernier (calliper) and measure a +/- 0.05mm variation.
I can not see that this tool will be of use in this case. Optical measurement (like tool-shops use) will be best I should think --- but who, I ask, has this at hand?

Greetings,
Axel