That's exactly why I mentioned "marked focusing range", because I realize that there are no such markings on G-lenses (is there a distance readout in the viewfinder? Never used G-system, so forgive my ignorance).
But you know the minimal focus distance for your lenses, which is already a good start (0.5 meters for 28mm and 45mm and 1m for 90mm).
Technically, since these are 35mm format lenses, unless there is some significant difference in design of G-lenses about which I'm not aware, bokeh would be the same as with similar SLR lenses. So, considering that you have 28,45 and 90, I would say that the best bokeh for the first two will be when your point of focus is in the range of 0.5 meters to about 3 meters, and from 1 meter to about 4 meters with your 90mm.
> I'm guessing the focusing range would be the
> shortest focusing distance through whatever
> lenght the camera reads for the lens before
> reading infinity
Well, let's make it easier.
Taking as an ex&le Sonnar 85mm f/2.8 again (I simply have it in front of me, not because it's so special), the marked range is between slightly less than one meter (about 3 feet) to 15 meters (50 feet) and then infinity. However, when you set your lens at minimal focusing distance, then change focus to say, 3 meters (approx. 10 feet), your focus ring rotation will be almost 180 degrees. But your rotation between 3 meters (10 feet) and 15 meters (50 feet) will be only about 70 degrees (didn't actually measure it, it's my visual estimate). So, the distance on the focusing ring itself will be the longest actually, but the real-life distance will be the shortest of the range for the lens, before it gets to infinity.
So, one third of marked focusing range on Sonnar (15 meters max before infinity) is from 1 meter to 5 meters (16 feet or so). In my previous post I gave an ex&le of 3 feet to 10 feet for this lens only because that's what I personally prefer, but that rough guide of "1/3 of marked range" usually works pretty well. And sometimes one-half is good as well.
Obviously in case of G-lenses it makes it a little more difficult, because you don't actually know the range of the lens. But since in your case you have the same focal lengths as ones available for SLRs, you can use data for SLR lenses as your starting point.
And of course, experiment and find what you like the most - tastes differ. "Diet or regular?"