I cut the my hair on by back to the length were I can hold it between my index and middle finger and trim to that. I don’t try to make it perfect and it seems to be OK.
I use the same 1/4" attachment for the back of my neck as I do for everything else. My mom keeps on criticizing it, and says I should trim it completely, but there’s no way I’d be able to get a respectable line between the trimmed-completely zone and the top-of-the-head zone.