[The HACK]

Back when I had my E46, I used to alternate ATE Typ 200 (gold) and ATE SuperBlue (blue), and on that particular car, it took:

~1/2L in rear passenger
~1/3L in rear driver
~1/4L in front passgener
~1/4L in front driver

Before the color would completely change. There's some intermixing, of course. It would go from gold to green to blue. Or the other way around.

This is without draining the reservoir first with a syringe or a turkey baster. I would surmise, with draining the reservoir, it would take a lot less for the first corner. Plus I'll bet all the intermixing wastes a lot of the fresh fluid in the flush.

Now when I "flush" the MZ4 Coupe, and since I don't use ATE SuperBlue anymore, and I use a syringe to drain the reservoir first, I usually go:

1/4 rear passenger
1/4 rear driver
1/8 front passenger
1/8 front driver

For a little less than 3/4 of a liter (or 1.5 bottle of Motul). And the fronts don't really require 1/8 of a liter, pretty much crack open and within a couple of seconds the fresh fluid would be coming out. This was done between OEM fluid and Motul, which is darker...Although, it's again, hard to tell when the fluids are intermixing. But any more than that, especially when you drain the reservoir first, would be a waste.

The reservoir takes a little over 1/4 liter, so the entire flush basically takes two full bottles of Motul.