I think 7qts is generally the amount that sits in the whole of the system, so including the torque converter, pipework, cooler etc.
What you want is a complete flush. For that most mechanics use special equipment, but you can do it by disconnecting 1 hose to the cooler and briefly crank the engine, top up (1/2 qrt or so), crank again, top up again etc etc. (on the z4 the transmission oil cooler is a heat exchanger on the radiator)
Like this (this guy does it continiously but be sure the transmission doesnt run too dry!)
Lots of videos on youtube on this topic.
You probably need 1,5x the total amount of fluid in your system to do a good flush.