Match the precision of your answer to the precision of the assumptions. Rough inputs usually do not justify a result with several decimal places.

Excess precision can make the answer look more accurate than the case actually supports. The interviewer generally cares more about correct setup, clean arithmetic, and a sensible interpretation.

If penetration is estimated at about 30% and spend is roughly $50, a rounded market estimate is usually more defensible than reporting $247.83 million.

State any rounding choice and sanity-check the result. Keep extra precision only when the case requires it for a comparison, threshold, or subsequent calculation. If you round, keep the direction of rounding in mind when the result feeds a later calculation so small approximations do not compound unexpectedly. For more detail, see the case interview math guide.

Rough assumptions do not deserve five decimal places.