Just like that, but offer a timeframe for a response. That is what builds trust and confidence.

You can also offer a "back of a napkin" answer if it's appropriate, but make it clear that you will give it the proper consideration and give a better considered answer. Maybe specify a large margin of error.

All that said, not everything requires rigor. In some cases, "fuzzy" answers are good enough. Learn to identify them and not to waste time giving less important things extra attention.