Get a formula that is actually right

For a chat window, Copilot.

The prompt

Here are the first rows of my sheet, including the header row exactly as it is:

<data>
{{ROWS}}
</data>

What I want to calculate: {{GOAL}}
Spreadsheet: {{TOOL}}

Before giving me the formula:
1. Tell me which columns you are using, by their actual header names.
2. Say in one sentence what the formula does.
3. Name one case where it would give the wrong answer.

Then give me the formula on its own line.

What to fill in

Placeholder	What goes there
{{ROWS}}	the header row and five real rows, pasted
{{GOAL}}	running total per customer, resetting each quarter
{{TOOL}}	Excel / Google Sheets

Why it works

Pasting the real rows rather than describing them is the difference between a formula that works and one that references a column you do not have.

The three steps before the formula are the valuable part. Asking for the failure case surfaces the off-by-one and the absolute-reference mistake, which are silent errors — the sheet does not complain, it just quietly computes the wrong number.

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