a flush is a high pressure process that can blow out seals inside the trans in areas that don't typically see high pressures.
It also typically introduces a bunch of foreign contaminants from the trans flushing system... such as metal filings from a previous transmission flush.
Drain, new filter, wipe off any metal filings that have been collected on the plug's magnet ... if there is a pan removal as well you can wipe that off as well, put it back together and refill and done.
If you feel like you didn't swap enough fluids you could always do it again after a few miles but I don't know anyone that would.
Yeah I'm not sure why everyone thinks you own a dealer's used trans flushing machine...they hear 'trans' and 'flush' in the same sentence and freak out. Lol.
Just my opinion, but since the trans takes 6 or 7 quarts, I'd just do a pan drop and refill, with a new filter. With that much fluid, there won't be much old stuff mixed in with the new. Plus you can clean the crap out of the pan and the magnet. Just make sure the car is level when pumping in the new fluid.
Boat, to be fair, the term "flush" is different than a fluid exchange, which is what the OP describes. From what most of us have experienced, "trans" and "flush" in the same sentence is worthy of complete freak-out.
I considered doing it with the lines and to large, new plastic buckets, but feared I'd not get the correct amount back in, or lose too much while reconnecting the fittings.
Do yourself a favor and put a drain plug in the pan. The fill plug is really unpleasant to access and use, while the engine is warm, as it's immediately next to the exhaust.
Next time, though, you merely turn a lever to drain and use an inexpensive, simple hand pump to put exactly how much came out, back in.