.38 away from the short walls -
If you have a room that's (for ease of measurment) 13x20, you're going to set up on the short side (13') and go down the long side (20').
The area of most "crap" - Null points, bad reflections - generally the worst possible place to be in the room is at the .5 spot - 10' from either wall.
The BEST spot in the room is going to be at .38 from either wall - 7.4 feet in. That will be the spot with the *least* amount of "crap" in the air. The lowest number of bad reflections, dips, null points, peaks, etc.
This is where bass trapping "in bulk" comes in handy - Especially on the rear wall - If you can fool the room into thinking that wall isn't really there (complete broadband absorption) your "sweet spot" will widend and lengthen considerably - In this room (where over 60% of the rear wall, plus the corners, are covered with broadband absorptive materials) there is little discernable difference between the "sweet" spot and the "sh*t" spot. In my old space (with much less bass trapping) the difference between the desk rail and the base of the chair was absolutely huge.
This is also one of the reasons why closets fail as vocal booths...
In a space where 4' is the long measurement, you're looking at having the mic in the sweet spot and the singer's mouth in the crap spot. Two inches makes a *huge* difference - and the problematic frequencies are mugh higher, with shorter wavelengths - where again, only an inch or two can mean the difference between "acceptable" and "complete garbage."