I'm not sure I understand your question but hope this helps.So the idea of draining the fluid JUST FROM THE 19mm plug at the end of a trans fluid service...
Can someone explain that to me?
How can I drain all the fluid from that plug...and still have the right amount of fluid?
The only way to replace all the fluid would be to have a transmission flush done. It won't all drain by removing the plug and stand pipe. When you replace the plug with the stand pipe and refill the transmission the correct level for the fluid is the top of the stand pipe at whatever temperature Nissan recommends.
You should have approximately the correct amount of fluid in the transmission with the procedure you used it's just that the new fluid mixes with the old fluid that was still trapped in the torque converter. If you wanted a higher percentage of new fluid you'd have to do multiple fluid drain/fills after driving a while and mixing the old and new fluid.
I plan to do a single drain/fill/filter replacement again at 60K just as I did at 30K on my Versa then between 90 and 100K I'll probably do 3 drain/fills and change the filters on the last of the series. This should replenish a large amount of the fluid to close to like new condition.