Calculate queries per second (QPS) step by step:
Given: million monthly active users, % are daily active.
Daily active users: M = million DAU
Daily actions: If each user performs actions/day: M = billion/day
QPS: billion seconds QPS average
Peak QPS: -x average = - QPS
Round aggressively. Use instead of . Interviewers care that you can estimate, not that you can do precise arithmetic.