Use commercial sense as a second check, not as a substitute for the math. If the arithmetic is sound but the result looks implausible, challenge the assumptions or units.

Case math is not finished when the calculator-like step ends. Interviewers want you to interpret whether the answer could make sense for the business.

Say that the calculation produces X, but X seems unusually large relative to the company, so you would verify the unit, time period, and key assumptions.

If those checks hold, keep the result and explain the implication. Do not change a correct number merely because it feels uncomfortable. For a related reference, see Case Interview Mental Math.

I think context matters a lot here. The same move can be fine in one case and clunky in another, so I would focus on the reason behind it.