Of the 2.4 million scouts, 1 in 5 is in a Mormon sponsored troop, or roughly 480k scouts. The assumption is that about 20% of those will remain in BSA in another troop (quite a number of those being ones close to achieving Eagle rank.) That’s where the 400k (384k rounded) number comes from.
True story:
When I was a kid, 50’s and 60’s, in small town rural Mississippi, the local Methodist church tried to get a BSA Troop going. They failed because the Baptists wouldn’t send their kids to an organization run by the Methodists.
Sounds crazy today, but that was what life was like then...........