"A Day in the Life" crescendo

(Straight Dope Classic, posted 8/9)
I heard that it was done by having every instrument play the scale, each one at whatever speed s/he chose.

“With enough courage, you can do without a reputation.” - Rhett Butler

Per Jeff Russell, The Beatles Album File and Complete Discography, “the orchestra build-up was recorded four times and then dubbed one on top of each other, slightly out of synchronisation…”

The worst thing that can possibly happen is not be used for something by someone - Kurt Vonnegut

I heard that the orchestra was given a sheet with two notes on it; a deep one at the start and a high one at the end (this would be consistent with the idea of playing the scale), and these instructions:

“Start here, simultaneously.
Stop here, simultaneously.
In between, do as you want.”