Cover the two modes: dividing a cost scope by total usage (e.g. cost per
order across the account), and per-customer mode (mapping a tag value to an
external customer ID your own backend already pushes via usage ingestion).
Important, easy-to-misread behavior to explain clearly: a ratio shows as
“—” / null rather than 0 when there’s no usage data for a period — that’s a
gap in measurement, not a real zero cost per unit, and the docs should say
so explicitly so it doesn’t read as a bug.
