In October, Kestrel & Vane makes $310,000 in total revenue and spends $62,000 on ads across Meta and search. MER is 310,000 / 62,000 = 5.0. Meta's dashboard shows 3.9 ROAS on $45,000 of spend, which answers a different question: what Meta takes credit for, not what the business made per ad dollar.

Platform ROAS can double count sales and shifts with attribution settings. MER cannot be inflated by tracking, so it shows whether more spend really brings more revenue. Its weakness is that it cannot say which channel or creative did the work, so use it alongside per-ad numbers, not instead of them.

Track MER weekly against a target set from your contribution margin. When you raise spend, watch whether MER holds: a falling MER at higher spend means each extra dollar buys less revenue. If you can, split out new-customer revenue for a sharper read on growth.