I spent more time than I should have at this stage in debating how to do the button band. This needle size or this one? This'll do X, but will it be enough, of should I go smaller/larger? And what kind of buttonholes, comparing techniques, pictures of them, trying to imagine how it'd look irl and guess which one I'd like.
The answer to all these questions? Swatches. It's always more swatches - remember, self! (Also I had to go actually acquire the buttons I wanted to use, to hold them in my hands & pass them through the test buttonholes, rather than trying to calculate based on measurements.)
In the end, I made three button band swatches, each 4-8" x 1.5", picking up along the edges of the one gauge swatch I still had. I tried out different needles, different bindoffs (also on multiple needle sizes), rate of pickup, and 4 different styles of button holes, then two of those buttonhole techniques in more sizes. And then I had all the answers, without testing them out in situ on the 125 stitch picked up button band!