AlanWS
You can say that the cap iron (chipbreaker, or whatever you want to call it) should be set on the order to the thickness of the shaving from the edge. This does not say the distances are equal, but that they are of similar size.

You could say that the chipbreaker should be set close enough to the edge that bending the shaving will apply downward force to the stock in front of the edge. Stated this way, it's easy to see that with a very thin and easily bent shaving, this distance will likely be very close to the shaving thickness, but as the shaving gets thicker and stiffer, the distance may increase more.

I think people focus on the short end of the range because that is seen as more challenging to set, and applicable to the most difficult stock.

I think it's interesting that these results emphasize the importance of being able to set a plane with a hammer, even though they show the utility of metal planes that are usually set by knobs instead. That's because the simplest controllable way to set cap iron position from the edge is using a hammer.