I went through all the 'how wide to make the tire track'-- reading all the op mans and TBN threads etc., with the net result that I still didn't know everything I needed to know.
Originally wanted the tire tracks as wide as possible for stability. However, it is recommended that the outer tire sidewalls be less than the bucket width, so that sidewall damage opportunities are reduced (for instance, when pushing through rocky soil with the FEL bucket). So the rears are as wide as possible but still less than the bucket width, a bit of a compromise since the safest for the sidewalls is to be as narrow as possible. For the fronts, the loader op man recommends as wide as possible (for stability), the tractor op man recommends as narrow as possible (to reduce front axle asm stress, and therefore increase front end longevity). So the front is a compromise-- I went with the maximum allowed by the tractor op man for the FEL load.
The only item that was not clear, was the relation of the front and rear track widths. From discussion with various people, the main consideration is scrubbing of the front tires in tight turns, particularly on hard surfaces. More scrubbing, sooner replacement of the fronts. Depends on what you are planning to do with the tractor.
In the end, the rear and front centerlines will be essentially lined up, rears with a few inches of clearance inside the bucket cut width (81") on each side. This should also allow chains if necessary (from factory, literally no room between the 3pt sway bars and the tires-- maybe 1/8" to 3/16" to the rims!). [the rears are to be widened several inches in the next couple of weeks, not a whole lot after all the investigation I spent on this. But at least I have some idea of why I did what I did, and why the stalky highest stability rear tire arrangement isn't recommended with my FEL and box widths, for what I use the tractor for.]