Hard to know without more context. It depends a lot on the trust between you and the asker. You'll be confident if you can tell that you are fulfilling their expectations on the whole. If that's not the case, it may be because the expectations are extravagant, or because reasonable expectations are not being met. What to do will depend strongly on the details.

That aside, I find it is useful to say a bit about what is involved in answering the question. "We haven't calculated that figure, but I can work it out based on these other measurements we have." "I don't know off hand. I know X colleague has some experience with that aspect, I'll check with them." "That's a little bit subtle because it might conflict with this other feature, I'll have to do refresh my memory." "That is relatively simple to do, I'll put it at the top of my list for when this meeting is over." (Of course, these also depend on the kind of question).