Tuesday’s set points
Derek reviews the other e‑mails he has received
on Tuesday and decides that 15 of Monday’s
messages must receive replies on Tuesday, giving
them a set point of 24 hours.  That means he has
the feeling they have aged to the point at which
the senders must receive replies or there’ll be
problems.
That leaves 42 e‑mails from Monday and Tuesday
in his in-box at the end of the day on Tuesday.