First, good topic.
The answer, I believe, comes from looking at it from the driver's perspective, and what we want to convey to drivers approaching from behind in our lane. Noisebeam and CB HI have already touched on this, but I want to make it clearer.
I believe our goal is to make it clear, in no uncertain terms, that sharing the lane (fully) to pass us is not an option (that overtaking drivers will have to encroach into the adjacent lane to pass).
And we want them to get this message sooner rather than later.
There is a
point of diminishing returns, but starting at the right edge each position 6" further left generally conveys the above message clearer and sooner. Exactly where the
point of diminishing returns is depends on the road and various conditions, but in general I find it is not reached until you're at least as far left as the center point between the left and right tire tracks. Beyond that, moving further left might still help in some cases, but usually not. But time after time I have found a significant difference in terms of conveying the message clearer and sooner by being near the center rather than even between center and right tire track.
The two clips in
this slide, by the way, illustrate what I'm talking about.