I was going to go with the fabric I was getting over 8 sts to the inch, then I did a quick stitch count for the body cast-on, and decided to go up to 7.5 sts to the inch, and save everybody a few dozen stitches.
You're totally welcome.
(Swatching for the inspired-by-Neville fair isle cardigan. Super fun. Knit Picks Palette in 7 shades of black, white, and grey.)