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Great post Ray.
When it comes to dithering and how much, it really depends on what you're doing it for. If you are dithering simply to reduce the FPN then you are better off having the values higher rather than lower (greater offset between subs). This stands to reason as even when doing random dithering there is a lower chance of two subs ended up in the same position.
If you are dithering because you are over sampling and wanting to drizzle more detail out, you then want a lower offset so that the pixel is just moving around a small area. An example of this would be a maximum of 0.8 pixel in any direction from the centre. This leads to each pixel mapping out 1.6 pixels in size with some slight potential overlap of surrounding pixels.
In essence, if all you want to do is reduce noise, it doesn't matter just as long as the subs don't double up. If you intend to drizzle out resolution from over sampling, I personally would make a max of 0.8.
Disclaimer: I haven't been able to test this yet, still waiting for new equipment and clear skies!
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