ROAS is lying to you: the metric stack I actually scale on

Every ad platform grades its own homework. Meta and Google will both happily claim the same conversion, and both will be wrong about incrementality.

The stack I scale on: contribution margin per order, blended CAC across all channels, and new-customer revenue as its own line. Platform ROAS becomes a directional signal for creative decisions, never a scaling trigger.

The practical fix costs a week: server-side conversion tracking, GA4 reconciled against actual store revenue, and one dashboard where finance and marketing read the same numbers.

When the measurement layer tells the truth, scaling stops being brave. You raise budgets because the margin math says so — which is a much calmer way to spend money.