For the bands I picked up 2 stitches for every 3 rows, which seems to be a good rule of thumb for getting a band that doesn't pucker or wrinkle. I ended up with 28 stitches on the peplum (the pattern called for 25) and 79 stitches from the peplum to the collar band. I had to adjust the spacing button holes (reduced the spacing by 1 stitch), and I ended up with only 7 buttons instead of the 8 the pattern called for.
MJ Trimming. They have a pretty nice selection of metal buttons.
Pattern: Margaret Beaufort, by Alice Starmore from Tudor Roses (2013 edition)
Yarn: Alice Starmore Hebridean 3 Ply (517 g in Lapwing)
Needles: US 4 (3.5 mm)