On most transmissions, you cannot drain all of the fluid out because of what remains in the torque converter and cooling lines. For this reason, the fluid "capacity" in a manual may not be correct to use as a measurement of how much fluid to put back in when doing a fluid change.
I have a graduated 5 gallon bucket that I use to measure exactly how much fluid is drained and then I replace the fluid with the exact same amount as drained.
This takes care of any fluid that may be in the torque converter, or cooling lines, and I am assured that I have not overfilled the transmission, which is as bad as underfilling your transmission.