Where did you get the bucket?To try to get out as much old fluid you can, a few has said you might get about 85%-90% changing the double drain method.
What I started doing after flushing about $120 down the toilet using transynd doing the double drain, I let my truck set for a couple of days up on 6” blocks, then removed the drain plug measured getting out 13 1/2 quarts, with the Allison deep pan, which should be around 75%, I had just did a drain and re-fill.
Drain your fluid into a measuring bucket, (pictured below) then you won’t have to be worried on how much to put back in.
As long as the Dex 6 is in decent shape with a pretty red color, I don’t think it’d be a problem.
If it’s pretty crappy and burnt, then maybe the double might be better choice.
Mines been changed 3 times (dropping the pan, not counting my double drain)
and I only have 84k miles, the inside of the pan and fluid has always looked very good.