Lewn,
The problem with 'trusting' the given dimensions for spindle to pivot is that it is expected that everything else will just automatically falls into place (dimensionally). Well, that isn't always the case. What you are trying to accomplish is for the stylus tip to correctly trace a given curve line. BTW: All tone arms with a cartridge mounted will make a perfect circle it they were being used to draw with. You want the stylus tip to follow a section of that circle (part of that curve), in which the record is going to be placed on. The setting up of the spindle to pivot distance only ''assumes'' that the stylus tip is then going to follow this curve. It just might if you only use the head shell that was or is provided by the tone arm manufacture. That is because he has provided those head shell ''slots'' to be in a position that would allow you to mount a cartridge so that it will follow that curve. Use some other head shell and those slots might be in the wrong place. I use the Mint LP Tracker or sometimes referred to as the Best Tracker as my cartridge set-up device. If you were to 'hold' that stylus tip onto that curve line, and you had the cartridge mounted in a head shell with very long slots, could you not move that pivot point in or our (closer to or farther away from the spindle), without moving that stylus tip? Yes,moving it out 6 feet would change the curve but we are not talking feet. Not even inches. We are talking 5 milimeters. The change in the curve would be even smaller that what the difference is between the Stevenson and the Baerwald. Not to even mention all the other variable alignment protractors that are out there! I find it more important to decide what head shells you wish to use, and then set up for it. Not the other way around and have your hands tied sort of to speak, being forced to use what ever head shell (slot position), that the manufacture decided upon. I like Magnesium for head shell material, and I like to have adjustable azimuth. Most tone arms do not provide this Azimuth adjustment feature. My Graham arm does, but neither my JVC 7045 or my Lustre GST-801 do. I do not want to shim head shells, so I have chosen the Sumiko's, or cheaper yet, the 'Zupremes'.
There is no industry standard as to where these head shell ''slots'' are suppose to be. This is sad but true.
The JVC 7045 tone arm came with its own head shell. I marked a dummy arm board (made out of plywood), per the spindle to pivot dimensions given and drilled it. I then mounted the arm, the provided JVC head shell with a cartridge installed. It followed the Mint tracker curve perfectly. I then remove the JVC head shell and mounted a head shell with a cartridge that was not a JVC head shell only to discover that I could not get the cartridge to slide far enough back in the slots to trace this curve. I tried several other brands of head shells that I have with cartridges mounted in them and none of them would allow me to position the cartridge so that it would trace that curve line. So where is the problem? Is it the spindle to pivot distance dimension? Well, if you only use the supplied head shell from JVC, then the answer is 'No'. But if you wish to use head shells supplied by other manufacturer's then the answer is 'Yes' . That spindle to pivot dimension is wrong. I did finally drill my arm board but I didn't use the supplied dimension given on the drilling templet. I drilled the hole so that my cartridges sit in the center of the slots when they are mounted in one of these Sumiko's. The stylus tip traced that Mint Tracker curve line perfectly. I don't know nor do I care what the spindle to pivot dimension is. I only care that it traces the curve correctly.
Regards,