The body's margin has to clear the band the zone renders at, and the
estimate counted paragraphs — blind to a line that wraps, to a run smaller
than the 12pt floor, and to the zone's padding. HeaderFooterLayer now lays
each section's zones out off-screen at their text width and reports the
height; the ODF zone's own gap to the body and its padding ride the
collapsed paragraph's spacing, which is where the export already puts them.
Measured on one file: a three-line band read as two, 14.9mm a page.