Stylus Rake Angle

I don't get this "slur" thing. The stylus is rigid/ inflexible and so whether the tip or farther up encounters the groove wall first, a deflection occurs. I'm not saying none of this makes a difference but if a guy who cuts lacquers for a living and a guy who builds some of the most coveted carts in the world says move along, there is nothing to see here, then maybe we should believe them.

On the subject of stylus overhang:
If the pivot to spindle distance is set exactly right, meaning +/-0.3mm of the recommended, because that is the best anyone can do, then it seems to me that small errors in overhang (say +/-1.0mm, but I would have to do the geometry before settling on that margin) can be tolerated.
Reason: None of the accepted tonearm geometries achieve more than two points on the arc of the stylus tip where there is tangency. The various algorithms differ only in the locations of these two points along the LP surface (and they also do differ in the amounts of tracking error at other points along the way, but I am not debating that here). A small error in overhang, assuming P2S is correct, will only move those two points of tangency by small distances in or out on the radius of the LP. So long as those two points remain on the playing surface, what is lost?

You answered your own question.

With regard to the distinction between SRA and VTA, there is also an equivalent distortion to Horizontal Tracking Error if the VTA is not that at which the record was cut. This Vertical Tracking Error is distinct from SRA and will influence tracking performance.

So with any given cartridge optimising SRA may mean VTA being compromised...

Atmasphere, you said:

As the stylus wears, sometimes you have to make little adjustments, like the stylus temperature. Funny thing- it cuts a slightly different angle depending on the temperature. Some LPs don't have very much in the way of dynamics so you can change groove depth a bit to allow for more time on the LP, conversely if something has a lot of dynamics or out of phase bass, you might cut a little deeper. So groove depth affects stylus angle too.

From what I understand, the cutter head angle may be a particular value, but the resulting profile on the record is different due to the springiness of the lacquer material which varies with temperature, such that the resulting optimum SRA is not that at which the record was cut. Variations in modulation mean that the cutter has to dig more or less deep, thus varying temperature and hence the profile, so that an optimum SRA at one point on the side may not be so at another. Would that be correct?

Lewm, you said:

A small error in overhang, assuming P2S is correct, will only move those two points of tangency by small distances in or out on the radius of the LP. So long as those two points remain on the playing surface, what is lost?

As you say, the important thing is the position of the two null points. Not because of their tangency, but because they define the distortion. As they become further apart, there is increasing distortion across the middle of the side. Closer together, the extremes are tracked with more distortion, particularly towards the centre. Small changes in null position make little difference.

However, that presupposes that the offset angle is set correctly which, rather than overhang or P2S, is , in my experience, the most difficult aspect of set up.

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Optimal SRA is an average based on the average 20 degree cutting angle standard, give or take. Temperature becomes moot as soon as changes in depth are introduced by modulation since that is accompanied by groove angle changes anyway. However as Atmasphere pointed out, average depth changes at the hands of the engineer and imperfect tooling set-up will certainly require a change in SRA. VTA refers to cutting angle. For all intents and purposes, VTA and SRA are virtually the same thing because it is impossible to perfectly maintain standardized specs. This is why the only way to match the two is by 'listening' and at best, listening on the fly.