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Concurrency follows Little's law again: peak-hour sessions multiplied by the fraction of an hour each session lasts. Daily active users on their own say nothing about concurrency, because a seven-minute session occupies only a ninth of an hour. Adding a 30 percent margin covers the fact that peak-hour concentration is itself an estimate.
Peak concurrency
daily sessions = DAU x sessions per user; peak-hour sessions = daily x peak share; concurrent = peak-hour sessions x session minutes / 60
Consumer services often see 10 to 20 percent of daily traffic in the busiest hour; internal tools can exceed 25 percent because usage is confined to office hours.
Both, as separate tests. Ramping finds the breaking point; starting at peak tests cold-start behaviour, which is what a failover actually looks like.