Frankenstein:
The MKIII NA tranny holds 2.5 US quarts, and I think the turbo tranny holds 3.2. You are usually safe to keep filling until it runs out the filler hole. That will, obviously, be enough.
The filler hole is towards the rear, a little above half way up the case, if I remember. It's the only plug there, and it's close to the transmission tunnel and a casting ridge, so it's not easy to get a ring spanner (box-end) on.