Back-of-envelope math only matters if it changes a decision

Plenty of candidates spend five minutes working out storage in petabytes and then never use the number. Estimate only what affects the design. Does the data fit on one machine? Does QPS need more than one DB? Can the hot set fit in memory? Round hard (a day is about 100k seconds) and move on. The interviewer wants to see the number shape the architecture, not to check your arithmetic.