Zendesk Support Reporting From Creation to Public Reply with Ticket Custom Field

Hi There,

we have a Ticket Custom Field from Severity (Sev1 ,2 ,3, Sev4).

We want to measure the time it takes from Creation to Public Reply and report per ticket and assignee the amount of minutes it takes.

Also, if later we downgrade the Sev 1 to Sev 3, which Sev will it report on at a later date? The first or the last Sev?

Thanks,

Adam