Measure before you promise
Almost every business sets its first SLA by guessing, and guesses optimistically. Run for four weeks with no target, then set the number near your current 80th to 90th percentile. You can tighten it later; you cannot un-break trust in the target.
Separate response from resolution
Set first response tight — that is the promise the customer experiences. Set resolution realistic and differentiated by category, because a password reset and a hardware replacement should never share a target.
Use business hours, or the number is fiction
A ticket arriving at 6pm on Friday should not be a breach by Monday morning. Configure working days, hours and public holidays before you switch targets on; this single setting causes more “the SLA report is wrong” complaints than anything else.
Two or three priorities, not five
Five priority levels always collapse into two in practice: urgent and everything else. Define two or three, give them genuinely different targets, and audit how often “urgent” is used.
Publish them
Internal targets nobody sees motivate nobody. Put them in the contract, on the website, and in a monthly report. A published target that is met is a sales asset.