The reason that automatic trannys do NOT have drain plugs is because people would drain the fluid and replace it and NEVER change the filter. This way they have NO excuse when the pan is down to JUST drain the fluid. THEN change the filter also.
Yes, loosening the LOWER line at the radiator and letting it drain into a empty oil or coffee can so you can see how much you drain out. Usually you don't need to start engine as pretty much will drain when you open the line.
GOOD LUCK