I won't claim to be an expert on servicing transmissions (though in hundreds of thousands of miles I've never had a transmission problem), but I've looked into servicing transmissions for Toyota's with the idea of servicing one for a family member. Haven't done it yet, but looking into the Toyota transmissions (measuring from the drain plug)... I'm impressed in a way with how they did it... but I feel like they tried to reinvent the wheel.
In trying to answer your question... I feel like it's relatively complex. Volume definitely varies from some ambient temperatures to operating temperatures so it could make a difference in how things turn out (another factor could be the shape and volume of the pain as well). With that said, I believe there is wiggle room so that you don't have to be 100% precise. But I would do my best to be as close as possible. With that said, I think it works to measure what you get out and put the same amount back in.
I can check later, but there is a professor from Weber State University I believe that makes some thorough videos on how to service some of these more complicated transmissions. He does an excellent job at explaining (and showing) procedures and specifics.
