Memorizing a small set of common conversions can help, but you do not need an extreme mental-math library to perform well in case interviews.

The biggest benefit comes from recognizing patterns you use frequently, such as common fractions, percentages, and multiplication facts. Beyond that, clean setup and reliable arithmetic matter more than showing off memorized shortcuts.

Knowing that 1/8 is 12.5% or that 15% of 200 is 30 can save time repeatedly. Memorizing obscure square values you rarely use is less likely to change your performance.

Master common fractions, percentages, and multiplication facts first. Then spend most of your practice on realistic case calculations where you have to choose the right setup under time pressure. If useful, this guide on case interview mental math gives more detail.