yes you seem to have got it right.
First make sure you can loosen the fill plug (if you drain it first and then find you cannot open the fill plug, you have a gearbox without oil, so can't even drive to a local mechanic.......for example!)
The drain plug should have a new copper washer before you close it up.
You will need a 12mm hex for the filler plug and a 22mm socket for the drain plug (well that is what I have on my 1990 S4 spider....guess they are the same)
The capacity is basically 1.85 litres, a touch under 2 quarts (as Redline comes in quarts, that is handy!)
Ideally, as you do not know what oil is in there, you should actually use a cheap oil to swish out the old stuff (like an SAE 40 "straight grade" engine oil, which is not dear) for a few hundred miles, then put the expensive Redline NS in....up to you. I did!
It makes life easier to get an oil syringe with about a foots length of clear hose on the end, to pump in the gearbox oil.
I bought something like this that holds half a litre........makes the job so much easier...redline is fairly thin oil and goes in a breeze with this type of syringe.
EDIT: yes fill it till it stops leaking from the fill plug - car dead level, obviously