I've been a contractor for over twenty years. Never have I seen such guarded secrets as I have with this subject.
I have applied "Knockdown" finishes with drywall mud, even cut it with water, latex paint, and sand texture. I don't think the texture can be applied properly without the correct mix and above all the right air settings.
Commercial applications generally use the "Cannon" which applies a nice high peaked mix to the wall or ceiling, allowing this to set for the proper time is also key.
Don't attempt to start to knock it down untill it's ready.
Are ther any books on the market that deal directly with this process?