Adding some things I want to the flam pages from Stick Control, 90 years after the fact:

Like an obedient little fool I once tried counting 20 repetitions while working from Stone— while standing at a concert snare drum in a disused hallway at the University of Oregon— then I realized that’s a stupid way to live, and now I just do them for a while. Set a timer for 30 seconds or a minute if you must.
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I always wondered where the Stone’s peculiar count of 20 repetitions came from as I refuse to believe it was just a random amusical number to keep us focused.
I think it was his way of making people understand they should do them for a long time.
It’s like Art Blakey’s lesson with Chick Webb, putting on a metronome and saying “now roll up to 100”, I think that’s more how people thought then.
https://cruiseshipdrummer.com/2011/04/03/chick-webb-drum-lesson-part-2/