Hi, from memory, I managed to get 7.25 litres out of mine by tapping into the return hose after draining as much as I could from the ATF drain plug.
If I were to do it again, I would do as you suggested by drain, fill, drive 500miles, then drain and fill again.
Much easier and less hassle, for marginal loss of ATF quality, really.
Be warned filling up through the ATF dipstick took over an hour of farting about, if you can make a contraption with a Big funnel and tube and make sure you can fix it securely under the bonnet whilst filling will save loads of arm ache!!!
Measuring/topping up the fluid using the dipstick SUCKS as it's not easy to read, so measure what you take out and you'll know how much to put back in.
Once filled, start engine and let idle, then move the gear selector one notch back at a time letting it rest for 10 seconds in each gear P-R-N-D-S1 and then back up to P 10 sec intervals again, repeat a few times to bleed any air out.
Hope this helps!