draining trans fluid



lets say you have a deep pan that holds 10 quarts of fluid. when you drain the pan, how much fluid left will be in the converter?
is it really necessary to take out all the fluid from the converter when you change trans fluid? I do mine once a year in the garage with the car under jackstands and just drain the pan, leave the car there for 24 hours then put the pan back on and fill. what do you guys like to do when you change the trans fluid?