Ok, fiddled around with it this weekend. I even took some scope pics.
I used a ziptie to measure. I did it 2 ways.
1) I measured the width of the threads of the fill hole (15mm) and bent the ziptie at that measurement to go straight down, so as to be snug against the inner wall of the bottom of the trans where the fluid sits.
With 2 liters in, I got a consistent 17mm to 18mm.
2) I measured it the way
@koozy does. More straight out with a curve at the end, with the straight part parallel to the ground when inserted. The curved part hits the oil and that spot is then measured from the straight part with the curve at a 90 degree angle (like he did in his diy dipstick thread).
This method was difficult, to say the least. Mostly because of the flexibility of the ziptie, and also it is difficult to get a clear path without a visual of the inside. Also, maintaining the dipstick in a parallel to the ground position, was near impossible without accidentally dipping it too far one way or the other. I could not get consistent readings, so I won't post the results.
As a result of the second method, I ended up adding 75ml, just to be on the safe side. After that, I was consistently well within the 20mm with method #2, even with variations.
One more thing, took some scope pics. The top of the fluid shows a reflection from a metal cross-beam. At 2 liters, it's just below it (tiny gap). I didn't scope after the 75ml additional oil, but I would imagine that the gap would not be there.
TLDR: I ended up adding 75ml to get me to 2.075 liters. I surmised that 2 liters is
mostly likely in fact enough, but added a bit extra for piece of mind.
*And as it turns out, 2.075 ltrs was the exact amount I put in when I did my first fluid change at 6,000 mi (OEM for OEM). I had the container I used back then with a line marked at the spot of the amount used. 2.075 liters exactly hits it.
[Edited post to add one more point & correct grammer]