The placement of both the cartridge box plate and the "eagle" shoulder belt plate was an "ideal" more so than a regulation distance or measurement.
For the box, the "US oval" plate was supposed to be at the visual center of the flap. For the "eagle," the visual center of the chest.
The accoutrement plates were NUG sent separately, and attached before issuance. Since there were no standdard location/postion measurements, one will find that they vary slightly. And, IMHo, and to my eye, the majority of originals have the center of the US oval plate below the visual center of the box flap something along the lines of say 60/40 down. (Plus that also varies between the older larger plate and the newer smaller plate.
This lower postion seems to have bee somewhat of a non-standard standard by iteself, for when the plates were eliminated with the Pattern of July 1864 box in favor of an emboseed US oval, the embossed version sits lower than the visual center of the flap as well.
Not only does the location of the shoulder belt palte vary, but the plate location will vary again by WHERE the lad wears the cartridge box on his right hip. Or, if he moves the box, backwards for "ease of marching" then the eagle plate will be lower on his chest. If he swings the box forward to be more visible in a picture, then it may ride higher on his chest. (or if the accoutrements are photographer's props, a lad may pose with a box that is too high or too low.)
And if the eagle plate was attached BEFORE the lad adjusted the billets for his height, body shape, and box location druthers, then when a lad adjusted the length of the shoulder belt the eagle plate would also shift.
A minor attempt to deal with this saw some shoulder belts being prestamped with slots at the manufactures, giving a lad the option of attaching his eagle plate higher or lower. But, since accoutrement plate makers did not always place their attachment loops at the Ordnance Manual standardized locations, precut holes did not always fit the plates issued.
In gleichem Schritt und Tritt, Curt Schmidt
Not a real Civil War reenactor, I only portray one on boards and fora.
I do not portray a Civil War soldier, I merely interpret one.