Consulting is all about collaborative problem solving. Interviewer needs to be involved in the discussion. However, key question is where? How can you construct a conversation to showcase 1. Your ability to build logic 2. Client friendly communication (involving interviewer). I suggest the following:
Step 1: Make sure that you know the objective of the calculation. Why are you doing it? Where will it fit in the larger approach.
Step 2: Communicate your business logic and calculation formula to the interviewer and ensure he is fine with it.
Step 3: Ask if he has any data/exhibit on it or you should make assumptions
Step 4: As per his guidnace in Step 3, proceed with the claculations.
Step 5: Try and relate the calculation answer to the business objective with which you had started in Step 1.
Hope it helps. All the best!