The AX4N transmission is definitely a weak spot on the Taurus Sable line. Purist and textbook would be to drop the pan, clean pan and magnet, change the filter and refill. I evacuate about 6 quarts on an annual basis and refill. Every third year I drop the pan, change the filter, and clean the pan and magnet. I have never encountered a dirty let alone clogged filter in 28 years on the Ranger, 19 years on the Taurus. Ranger went 269,000 miles, 2000 Taurus closing in on 150,000 miles. I pump out the fluid before dropping the pan as I find it a lot less messy. Even with a large drain pan, being careful usually spill transmission fluid on driveway, my hair, etc. I would not fault anyone for the pan drop, clean, and filter replacement. A lot of these shops doing a flush do not drop the pan, nor change filter. I think it is great people are considering changing fluid, filter, etc. on transmission as some want you to believe it is a lifetime fluid. I agree that if someone just suctions fluid and replaces that is a half measure, absolutely. The original poster asked how much fluid could be removed via suction. Answering his question makes you an half measured maintenance person? Do what works for you.
Scott