OK, so based on the following spec: > It is recommended that the reference pixel be the visual angle of one pixel on a device with a pixel density of 96dpi and

OK, so based on the following spec: > It is recommended that the reference pixel be the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length. For a nominal arm's length of 28 inches... It would seem that the reference pixel is simply 1/2688 of the typical distance between your eyes and the device. If a device is meant to be used at half the "arm's length" distance (14 in), the reference pixel on that device would be only half as large. If a device is meant to be used at 3x times the distance (84 in), the reference pixel would be 3x larger. Much easier than angular diameters.