I'm doing more and more with pieced backs just for design reasons which sometimes include I didn't have enough fabric. There are different ways and thoughts.
In general, for a backing fabric seam, you use a 1/2" seam and you press open. You do want to cut off the selvedges and not include them in the seam allowance if you can, they are a bit thicker and rougher and it adds up, plus there are tensions in the fabric that aren't released until you cut them off.
You want to avoid a seam being right on a fold line, that is particularly down the middle in either direction. The natural tendencies to fold the quilt there also tend to be hard on the seam. Just a couple of inches to the side is enough of an off-set. Many long-armers prefer the seams to go across horizontally, that was never much of an issue for me.
Sometimes with the shape of fabric and our projects you can cut a piece along a diagonal and sneak out a few extra inches that way. All sorts of diagrams and such, but here's a good place to start with John Flynn explaining it.
https://www.flynnquilt.com/free-lessons.htm
Here's a pdf (non calculating) from Jinny Beyer using John's math, shows a diagram
https://jinnybeyer.com/wp-content/up...-Worksheet.pdf
And here's someone else with a video references those!
https://www.youtube.com/watch?v=4ebZG0CReGM