The URL at the bottom of my post is not a complete answer to your question, but I found it to be interesting.
I do not own an MB with a 722.6 transmission, but from what I've read here, you may get the usual "filled for life" BS. That's a story the dealer wants everyone to believe. It's basically a line of you-know-what designed to sell cars to the majority who prefer not to maintain their vehicles.
I wouldn't worry about the "quoted" capacity. Do this.
Drive the car for 15-20 miles to fully warm up the transmission. Afterwards, do the dipstick check and note the level. If it's full, skip to the next paragraph. If not top off in slight increments until it is.
Shutoff motor and drain pan and converter, although from what I've read, you may not have one. That shouldn't be hard to determine. In either case, drain and capture all fluid. Also catch what comes out when you pull the filter. There will be more.
Now measure it....all of it and pour that amount back in after you replace the filter and refit pan and all drain plugs.
That's all there is to any transmission fluid change with the exception of units w/o drainable converters. If that's your situation. drain what you can, change filter and move on. Automated flush systems have been known to deep-six transmissions as they break debris loose and a miniscule amount of debris will kill a transmission. This sort of thing is more likely to happen to someone who decides to flush a unit that's been neglected for years.
Needless to say, one must use the correct fluid for the unit being serviced.
http://www.peachparts.com/shopforum/showthread.php?t=106649&highlight=722.6