I finally get it!

First and foremost, every time I see Phoenixengineering participate on this Board I smile. He is a great all-around guy and has a wealth of useful knowledge. 
Now that aside, as someone who gave up on VPI decks (three of them) and went to vintage decks on custom plinths and Reed 3P arms, I have learned from personal experience that P-S is not a demanding spec if you have a typical headshell that allows the cartridge to slide back and forth. I have learned from mounting my own arms that a tonearm's manufacturer's specified P-S may not allow for all modern cartridges to come into optimum alignment. You may argue that this is the fault of the tonearm manufacturer but such is not necessarily the case-often it is due to the cartridge manufacturer having an unusual stylus to cartridge body mount configuration. 
Case in point would be my current VdH Crimson Strad. When I mounted my 10.5 Reed 3P to the spec'd P-S, I could not move the cartridge far enough forward to get to the Lof B on my Feickert alignment grid. Remounting the arm a few mm's further away from the spec'd P-S allowed me to the to that point. 
Brian Walsh of ttsetup.com confirmed that what I had done is the only approach and that it does not, in any way, compromise performance-he was able to accomplish optimum alignment with his very sophisticated software based tools. 
So imho HW is correct that being a mm or two or even three is not a problem if the stylus can get to the desired point given your alignment type/choice. To put it slightly differently, the stylus does not know or care about the P-S distance. P-S serves as a range within which most cartridges can be optimally aligned within the slots. I have said this before and will say it again-the SME approach to P-S proves my point: no slots and instead the P-S is changed for each cartridge using their ingenious pivot point sled. 

Exactly that I am trying to say.
Excellent idea to move the pivot point instead. 
So the spindle to pivot length is nothing to OCD about.

The just make sure that we can hit both of the points on a arc protractor without moving the protractor or turn the platter.

(If you do it by moving the pivot point or sliding the cartridge back and forth in the head shell just don't matter.)

And we are done with a alignment.

oldears has got it right. A 1 mm difference in pivot point can easily be accounted for by any protractor with the exception of the expensive "arc" ones that are adjusted to the theoretical distance rather than the actual distance.

Theoretics aside, I have always believed that the adjustments for horizontal tracing angles (for which there is no universally accepted standard--there are many and all are compromises) are obsessed over.  Not to mention the hundreds of dollars spent on protractors that give the same result as a $20 one.  Proof of this, I suggest, is found in the success of the Viv Lab Rigid Tonearm, a short arm with no offest.  Said to sound great.  Tube units measure worse and sound better.  Same with LPs and digital, same with belt drive over most DD.  The only test is your ears.

More important cartridge adjustments IMO are SRA (depending upon stylus shape) and especially azimuth.

Right. I wish more would realize this. Not as much for their own sake. Anyone wants to spend more money on protractors than stuff that actually makes music, and more time obsessing and fussing over microns when the dang thing ain't within millimeters most of the time anyway, that's all fine with me. To each their own. 

But unfortunately new people come along and see this and conclude playing records means having to do all this crap. Happens all the time. Then they either sit there looking at the ten grand rig waiting to pay some dude to come set it up, or decide its just not worth the hassle. Which is a shame. Because its not a hassle. Its easy. But who is gonna believe that when the guys who want to make everything just as hard as it can possibly be are sucking up all the oxygen in the room?

A arc protractor is free.
As downloaded SW and you put in your specific TT S2P distance.
And you chose what mathematical equation that you want (Löfgren A/B, Bergwall or ..)
Print it out, check distance of the printed result to ensure it is in scale.
Done.

As for azimuth I were one of them that obsessed to get my setup as good as possible. After reading people setting up with special test/calibration records. And with special equipment (fozgometer) or computer software. I also used digital oscilloscope and so on.
Read people that had many different pressings of Denon calibration records + other brands of calibration records. And they come to the conclusion that every single calibration record measure slightly different... And remember supposedly when making a calibration record they hopefully make all the angels setup on the cutting lathe as good as possible.. and we end up with variance anyway..

So should we set up perfectly against one calibration record? What is then the result.. .. that we only have a perfect setup on THAT record that mimics just that particular angle that the cutting head had when it cut just precisely THAT laquer..

So the lesson was azimuth vary from album to album. And if we suppose that the cartridge manufacturer has done as good job that they can do. Then it is just to put your cartridge for azimuth as horizontal as possible. And in a average between albums it will be the best.
(If it is not damage/faulty in any way)