Rule 1 of 13 · Chapter I — Before You Point at a Number
Write down the outcome you actually care about in plain language before you choose any number to stand for it.
Why this rule exists
A metric is always a proxy, and the gap between the proxy and the real goal is where all the later damage lives. If the goal was never stated, nobody can see the gap widening, and arguments about the metric turn into arguments about nothing. Writing the outcome first also exposes the cases where you don't agree internally on what you want — far cheaper to discover now than after a dashboard has been built on the disagreement.
In practice
Write one or two sentences, in ordinary words, naming who is better off and how, with no metric nouns in it: "customers get their issue resolved on the first contact and don't have to chase us." Get the people who will be judged by the measure to agree the sentence describes something worth doing. Then keep the sentence attached to the metric everywhere it appears — in the spec, the dashboard, the review deck — so the proxy can always be read against it.
When it doesn't apply
Exploratory measurement, where you are watching a number to learn what is happening rather than to steer anyone, doesn't need a stated outcome yet; it needs one before you attach any decision to it.