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Compute CAC payback on gross margin including free promo months
- For FP&A and financial analysts
- You practice Catching mistakes and made-up facts
- Time about 5 minutes
- Level Intermediate
About this exercise
You're calculating CAC payback for the CMO. Write a prompt for plain text with lines starting "Monthly margin:", "Paid months to recover CAC:", "Free months:", "Payback:" and "Versus marketing's figure:". Use gross margin dollars, not revenue. Free promo months earn nothing and count toward the payback time. Show the math. Under 90 words.
Common mistakes when you ask AI for this
- Using revenue. The AI divides CAC by $1,000 and gets 12. Try: Say: "Use gross margin dollars, not revenue."
- Ignoring the free months. The AI stops at 16. Try: Add: "Free months count toward the payback time."
- Skipping the comparison. The AI leaves out marketing's figure. Try: Add: "Versus marketing's figure: the difference in months."
- Markdown extras. The AI adds bold labels, headings and a closing note. Try: Say: "Plain text, no bold. Output only the lines asked for."
- Trusting the AI to count words. AI is bad at counting and runs past word limits. Try: Ask for fewer words than the real cap and keep each part short.
What a strong prompt includes
- Names the five lines
- Uses margin
- Adds free months
- Asks for the math
- Caps length
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