Hey guys. Yes, I think it was I who started the whole 38% thing as a starting point for the listener position. But I never called it a "rule" - that just sort of happened as it spread around the internet.
The main point is to get safely outside of the midpoint (50%) of the room, and 38% often works out to be about right, particularly when using wall-mount speakers in a typical mid-sized (20 ft. deep) control room. For a mastering room, approx. 38% from the back wall might actually be better, if the room is deep enough that 38% is 10 ft. or so.
For an all-in-one room like the one in the EQ article, there are many considerations other than just the straight up question of where to place the listener in an empty room. You've got to have room to accomodate performers, room for a couch, and so on. So it usually makes sense in that type of scenario to place the console and listening position forward of the 50% location. And again, 38% is recommended as a starting point, not a "rule".
I often use RPG's Room Optimizer program to check the interaction of the modal and boundary effects, and combine that with knowledge gained from experience of past room designs. Then there's the size of the sweet spot, and the interaction of the speaker focus point with the room modes to consider. All of these things go into the equation when deciding on the listening position. But all things considered, 38% is not a bad place to start.
--Wes