Write enough to make the setup and logic easy to audit, but you do not need to document every piece of arithmetic. The interviewer should be able to follow the equation and the important intermediate numbers.

A good rule is to write the formula, the key inputs, and any calculation where a mistake would be hard to recover from. Simple arithmetic can often be done mentally if you are accurate and comfortable explaining it.

If writing is slowing you down, practice the same calculation twice: once with your normal notes and once with only the essential steps. Keep the minimum that preserves accuracy. If useful, this guide on case interview mental math gives more detail.