Use your best judgment and make sure that every line item in your estimate is defensible. "I used the best information that I had access to, happy to add your i

Use your best judgment and make sure that every line item in your estimate is defensible. "I used the best information that I had access to, happy to add your information if it's better" is a good response to a challenge. Surface uncertainty rather than hiding it, and fight for the time to resolve uncertainty before committing to a timeline. Fight to shrink the assignment if it seems too large.