Cooler lines, yes, sort of, if you have it open/draining for enough hours, it drains down. You can get more than a quart+ if you can leave it overnight etc. even more if it was not running just before you drained it.

Assuming the level was correct before draining, either way, you'll have the same amount of fluid in there now as you had previously. Run it for a month, do the same procedure again.

 

After doing the above trans fluid drain/fill, I’ve made several normal day to day drive cycles getting trans up to temp, I’ve checked the fluid level and reading in the upper part of the “B” section of the dipstick. This is the same before I did my drain/fill.

this was a really easy and clean process, my plan is about once a year going forward.

 

Kudos on adding a pan with a drain plug. I just fitted an aftermarket plug to the existing pan on my first service.
Once you have the pan empty why not just drop it, change the filter and clean the pan?

2012 F150 Eco-Boost w/185000 miles going strong.

 

the idea is doing a drain/fill on the fluid every 15-20k miles. The filter doesn’t need to be changed that often, and only a portion of the fluid is changed whether you have a drain plug or drop the pan.