Spreadsheet formula helperPromptLow risk

I'm using {{spreadsheet_app}}. My data: {{data_layout}}

I want to: {{goal}}

Give me:
1. The formula, ready to paste, with cell references matching my layout.
2. A plain explanation of each part.
3. A version that handles blanks and errors gracefully.
4. If there's a simpler approach (pivot table, filter view, QUERY), mention it.