Ok, so I looked it up.
David F. Butler in 'The American Shotgun' has a a brief blurb on exterior ballistics. His interior ballistics dissertation is much more extensive and highly recommended.
Butler was an 'engineer' at Winchester.
He says in reference to a chart that starts with 1336 fps at 3 feet (coil measurement), "This equates to slightly over 1350 fps at the muzzle".
So, vague and imprecise but no assertion of 100 fps loss in 3 feet.
So, he's saying a loss of about 15 fps in the first yard. How does that get us to a loss of 400 fps at 20 yards?
I have a book called "Shotshells and Ballistics" by John Taylor. I don't know the source of his figures, but he lists velocity--both muzzle and standard American 3 foot, out to 70 yards--for the majority of factory loads available at the time the book was published (2003). He gives true muzzle velocity of a 1330 fps load as 1436 fps. Velocity at longer ranges are similar to the numbers Miller posted.
