The pauses you should subtract
If you are waiting for information from the customer, a part from the supplier or the agreed maintenance window, that time should not count against you. An SLA without pauses turns the metric into a lottery and discourages exactly what you want: asking for the missing information.
But pauses can be misused too. If a large share of your cases is always “waiting on customer,” that label is probably hiding stalled work.
The average lies
Use the median and the 90th percentile. An 8-hour average can hide the fact that one case in ten takes three weeks, and those are exactly the ones that generate complaints.
The usual mistake
Closing cases to improve the metric, only for the customer to reopen them. Always look at resolution time alongside reopened tickets: if one goes down and the other goes up, you have not improved anything.