Write a spreadsheet formula

Help me write a spreadsheet formula.
Inputs: goal; sheet layout; app.

1. Define row grain and missing-value rules.
2. choose compatible formulas and explain ranges.
3. verify normal, boundary and error examples.

Deliver: Formula, How it works, Edge cases, Alternatives.
Clarify critical gaps, separate facts from assumptions, and do not invent data, citations or completed actions.

My material: [fill in here]

Before you start

Provide goal; sheet layout; app.