The more I look at the more I believe the 400%/320px measure is near perfect.

Alastair really hit the mark with this calculation. Here is why I think so.

User Side
1. Each screen has an optimum viewing distance for each person. As Alastair
points out, screen manufactures try to make the mean of these optimum
distances be the distance at which 16px = 12pt. Where pixel is a screen
relative measure, and point is an actual measure 1/72 inch.
2. A person with low vision does not have to view the screen from the mean
optimum distance for normal readers. I usually view from about 2/3 that
distance. The effect of my moving the screen to 2/3 distance is to increase
the effective font size by 150% that is 450%. Of course cutting the
distance in 1/2 with 400% gives a size increase of 800%. I have tried these
distances and they both enable viewing the entire screen.

The net effect is that a point size of 12x8=96pt can be achieved with a
person retaining a healthy posture.

Developer Point of View
1. Sizes can be computed in pixels without loss of generality.
2. The methods to achieve 320px good behavior are extremely well tested.
3. A large number of websites meet this goal already.

Maybe a few of us should combine this with some more good things about the
number and publish it in A List Apart.

It might be a good way to sell the change.

Wayne