"with the vast majority of this loss coming within the first 5 yds."

Well, no... it doesn't say that at all.

It does say... that roughly the same velocity loss occurs in the first 20 yards as does in the next 40.

Backing up to the muzzle, with the faster load, 20/3 is 6.6, so at 6.6 yards we should have lost 1/2 of 405 which is 202.

I'd guess 170 fps loss at the 5 yard marker, which is only 22% of the total loss, not the vast majority.

I get the point that it's a V squared issue, and that drag is a bit more when above sonic, but I ain't buying that we lose 100 fps the first yard. It's more like half that, and choke effect is known to add 50 fps anyway.

"The price of good shotgunnery is constant practice" - Fred Kimble