Use the calculator when it reduces error or cognitive load, not automatically for every operation. Simple arithmetic may be faster mentally, while multi-step calculations can be safer with the tool.

The important part is avoiding unnecessary switching. Decide quickly whether a calculation is simple enough to do directly or complex enough to justify the calculator.

Practice with the same decision rule so you are not debating the tool choice during the assessment. If useful, this guide on McKinsey Solve guide gives more detail.