For my PM, purchased, new, I was simply going to assume the factory fill was correct and then drain out what I can (drain plug in pan method or evacuator) and then replace with the exact same amount of fluid that got removed.
If anyone knows that this simpleton's method is incorrect, please let me know. I still have a lot of driving before I hit say 30K miles or so before I plan to do this so lots of time to think about it. But my Caravan, same transmission as PM, is getting due soon (90,000 Km). Due in my mind that is, because its manual suggests that transmission fluid is lifetime (no replacement necessary) and I'm thinking "er...no.".