I'll weigh in as a restorer, not as a maker. Personally, I use the 1/7 method to start but I also reference 19.5 from the center at the stop (measured between the notches). I start with a bar length of 7/9 the length of the top equidistant from the ends. I prefer 1mm inside the bridge foot. I think any of these methods are useful and when they are carefully considered, they are a sharp tool. Everything that lands on my bench is different and so I always ask myself "is there a concept here, on the part of the maker and/ or on the part of the person that last set it up (inc the bar)". Next question is how well does it work? Then what can I do to balance out some weakness, or enhance a strength. Ultimately, I am looking for a physical structure that works efficiently, so that the player's experience is that they are handling an instrument that is willing.
1/7th of half the width or 1/14th of the entire width is like the saying, "six of one and half a dozen of the other". Sacconi, in his calculations, took his measurements from the outside rib line and not from the edge as the edges are often worn in older instruments. If you look in the Sacconi book on his diagram you will see this. This is intended to give you an angle and only an angle. If Sacconi in wanting to show how he oriented his bass bar needed a position slightly further out or in he could just as easily have used finer increments such as 28ths, 56ths, 112ths, or 224ths for the divisions; all of which will give you the same angle. He naturally reduced the fraction for working convenience and mathematical convention. I don't think that anyone is obliged to use this sytem but it is interesting to note that Sacconi's observations stem from footprints left by the original bassbars along their gluing surface. It tells us something more about what Stradivari was doing and this in itself is interesting to me.
In the end I think David is underlining that setup is NOT JUST THE TABLE but the WHOLE INSTRUMENT from upper nut to the end button. To take into consideration how to position or align your various components between these two points is to have a complete overview of your setup.
I find that when all other things are "right", then very small changes in things like bass bar placement (or neck angle or bridge cut or saddle height...) can make a big difference. I also think that the placement of the bar will greatly influence the future adjustability of the instrument in terms of set up - bridge,post etc.
Rules aside, I will move a bar laterally (especially depending on the arch) or change the slant slightly, trim wood off either end of the length, vary my shaping scheme, choose a stiffer or softer piece of bar stock....etc. I am talking about variations really very small in scope that I hope in combination will support my idea of what the instrument needs.
Ahhh, bass bars. One of the endless discussions!