Calculate when the magnitude changes the insight or decision. Do not calculate just to turn every visual observation into a number.

Exhibits are testing prioritization as well as arithmetic. Some differences can be described directly, while others need quantification to support the conclusion.

If one segment is clearly twice the size of another, you may not need an exact percentage. If the client needs to know whether growth exceeds a 10% hurdle, you do.

Before calculating, ask what decision the number will help you make. If useful, this guide on case interview insight generation gives more detail.