Terms, without 'it depends'.

What a customer spends per order on average. The lever to pull when advertising gets more expensive: a 10% higher AOV means you can bid 10% more per click without losing margin.

What it costs to bring in one new customer — counting all marketing costs, not just ad spend. A healthy CAC is one you earn back within 1–2 orders.

The percentage of customers (or subscribers) you lose in a period. For webshops without subscriptions: the share of last year's customers who bought nothing this year.

The percentage of visitors who actually buy. Dutch webshops sit roughly between 1% and 3%; above 3% you're doing something right — or your traffic is very warm.

The total profit one customer brings you across the whole relationship. Together with CAC the most important ratio in ecommerce: LTV ÷ CAC above 3 is healthy, below 1.5 you're slowly bleeding out.

The share of orders that comes back. Fashion sits at 20–40%, most other categories under 10%. Subtract returns from your revenue before you calculate ROAS or LTV — otherwise you're kidding yourself.

Revenue per euro advertised. Careful: ROAS says nothing about profit — a ROAS of 4 on a 20% margin is a loss. So always work through to POAS (profit on ad spend) or use your break-even ROAS as the floor.