Contribution LTV over a stated window divided by nCAC - the playbook's Gate 3, the value screen on customer acquisition. It answers whether the customer is ultimately worth what you paid. Two traps. First, the famous 3:1 rule came from SaaS, where 80%+ margins and recurring revenue make a long horizon safe to bank; importing the threshold while dropping those assumptions is the actual error - run the ratio on contribution LTV at a window you can observe, never a retention curve you are hoping for. Second, the ratio ignores time: two brands with identical LTV:CAC can have completely different survival odds, so the cash gates (first-order floor, payback) bind first.
Benchmark. At a 12-month contribution LTV window: nCAC at 20-33% of LTV (3:1 to 5:1) is strong and scalable; 33-50% (2:1 to 3:1) is workable only with payback inside 6 months. Run it across cohorts and it becomes a budget-ranking engine, not just a filter.