Okie - A bunch of the fluid is sitting in the converter.
Mike the idea that you're doing anything other than contaminating a bunch of new trans fluid with this approach made me smile. The idea of a math based solution to figure out how badly you are contaminating it made me spit up Pepsi on my computer I was laughing so hard.
Oh I hear ya. I hate it too. On the Honda Odyssey that is the method that is RECOMMENDED by HONDA. They say to do a drain and fill every 30,000 miles which in the case of that vehicle is 2.5 quarts to 12 quarts. So the contaminating the new fluid with the old fluid is straight from the manufacturer.
Let's jump to TOYOTA now. They also have a drain and fill method that is outlined by a 12 page TSB that my friend who works at Toyota sent me. It requires a thermal temperature sensor and some other long drawn out procedure. Either way they also had the contaminated fluid procedure.
I wish that is was a clean drain it all and put all new fluid in there but they (as you said) want you to bring it in to them and I think it is absurd to have to spend $380 to do a simple fluid change.
I say all this and when I bought the Toyota Sienna was told that the reason Toyota did not even have a dip stick to measure the fluid was that they placed LIFETIME transmission fluid in it that never needed to be changed. I said BOGUS on that. Lifetime until it blows up due to not changing the fluid is what I say.
So, I figure that the contaminated fluid change and doing it a couple of times is better than the "leave it in there until it blows" method.
Anyway, I found out wha the formula is. R = (9/12)n (that is to the nth power didn't know how to type it) 9 is what is left in the system (dirty fluid) and 12 is the total capacity of the system. So if I changed the fluid 3 times there would be 42.1% old fluid left in the system.