How to set SRA after determining true vertical?

If you have a spherical stylus or even an elliptical sytlus, then the actual SRA is not too critical because whether the stylus is vertical, or leaning (less than a degree) slightly forward or backward, the very rounded tip is going to fit in the groove more or less the same way in all cases.

Such is not so for (the more common today) "micro-ridge" type styli, pioneered by Shibata and van den Hul. These styli are shaped a bit like a spade shovel, broad across the groove and thinner front to back. Their long edges touch the groove on each side, in a line running more or less from top to bottom.

The cutter head is VERY spade-shaped -- a chisel actually. It is not vertical either -- never has been, even with 78s. Why? Because if it were vertical, the cuttings it produces would jam between the head and the groove and ruin the master. Instead it is tilted forward (at the top, just like you'd do with a wood chisel) so that the cuttings keep "peeling" off and out of the way.

Therefore the "wiggles" the cutter cuts into the groove are all "leaning" forward as well as wiggling side to side.

If you have a "cutter-shaped" stylus (micro-ridge) and you want it to make the best contact or "fit" with the groove, it has to be tilted forward so that its edges match the forward-leaning groove wiggles. This angle of tilt is called the Stylus Rake Angle, or SRA. It is usually between 1 and 2 degrees for most recordings. But as Doug already pointed out, there is no actual standard, and cutting lath operators have some leeway in how they want to adjust the head for a particular recording, or perhaps use with certain brands of blank masters.

This is why I recommend adjusting the SRA to just under 1.5 degrees. Then, if you have a tonearm that's easy to raise or lower, you can "fine tune" the SRA for a given record -- increasing it for a thicker record, for example, or decreasing it if your ears tell you that maybe the lathe operator set the cutter closer to 1 degree.

A "vertical" setting would never do for a micro-ridge type stylus, because the edges of the "spade" would be straight up-and-down, and would actually scrape across the tilted wiggles instead of fitting into them.

Nsgarch- you make a convincing argument. your shovel and chisel anology could be true but they move in only one plane. the cutting lathe moves in two planes. I believe your analogy would be true if the cutting head were like a jackhammer titlted forward. Then each groove would be cut from the top with the resulting angle of the jackhammer. Not so when the cutting head is moving in two planes.

Like I said I definitely hold the minority opinion.
BTW thank you for such a gentlemanly rebuttal.

Greg, actually, the chisel, or spade-shaped cutter, can only move in one plane. It's a plane that lies across the two "spade" edges. It moves diagonally (up left or right or down left or right) but remains constrained on that plane. It's movement is created by two electromagnetic drivers on each upper corner of the cutter blade. It can cut a signal (wiggly undulations) into either groove wall while cutting a smooth groove wall (no signal) on the other side, Or it can cut signals, even different signals, into both sides of the groove at the same time.

I wish I could find a decent diagram somewhere to post. If not, I'll try and whip something up and post it.

cutting in one plane would dig a hole. to make a 45 degree grooved trench you have to move in two planes n'est pas? maybe I stated it wrong.

Greg, yes you simply stated it backwards. The cutter head stays in one plane but moves in two directions.

Imagine you have a nickel on your desk. Imagine that's the cutter head looking from the front. You put your finger on the nickel. You push it up and down, left and right. But it still remains in one plane -- the plane of the table.