Treat the challenge as a signal to revisit the assumption, not as a demand to defend it at all costs.

Briefly explain the logic behind your original number, then adjust if the interviewer has identified a legitimate weakness. If no replacement is given, choose a revised assumption yourself and explain why it is more reasonable.

For example: "I used 30% based on a broad target group. Given your concern, I would narrow it to 20% to reflect actual purchase intent."

Keep the sizing moving. The interviewer is often testing whether you can incorporate feedback while maintaining ownership of the model. If the revised assumption materially changes the final estimate, call that out so the interviewer can see which conclusion is sensitive to judgment.