Transmission fluid quantity

My 69 FSM calls for 15.5 pints in a 904, 18.5pints in a 727, with some exceptions.
Trying to drain 2.5 pints will be a nightmare,so
sneak up on the full line.
I am getting the idea that I should take it slow and easy filling it up. I am not in any rush as I am just about ready to get it running but still have no interior so will be doing all this up off the ground for a while.