I tend to pull my working threads tightly, so I thought I might show a small trick I've worked out for doing very narrow satin fills, two threads high, that helps keep them even:
I "park" a spare needle across the threads to be covered before I start, and then work the satin stitches OVER the needle. (There are two separate stitches in each opening rather than working with a doubled thread.)
This lets me snug each thread gently up next to its neighbor for nice, even coverage. And I can tug firmly on the working thread because the spare needle is reserving some breathing room in the row of stitches. Once the spare needle is pulled out, there's a tidy little row of satin left in the space.
Rachel Jarratt, Band 4, done.