If you really want to flush the transmission I think a better way of doing this is not to run the engine continuously.
I did a tranny flush on my wife's previous car, and after dropping the pan, changing filter and filling it back up decided to flush it.

I disconnected the transmission cooling line running to the radiator, put it in a bucket with volume markings and had my wife start the car. I'd yell at her to shut it off after collecting 2 quarts (which really only took like 10 seconds at the most). I would dump 2 quarts of fresh fluid back in (which really took A LOT longer then 10 seconds..) and rinse and repeat until what came out of the line was nice and red.

This way you're not running the transmission too low. Just a thought. I think it took me a good 5 gallons of tranny fluid just to flush everything out, and it kinda felt like a waste to be honest. Flushing out perfectly good fluid like that. There has to be a better way, but I wouldn't know what.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)