Average Resolution Time Report

I've been asked to create a report which shows average resolution time in hours for cases in my org, but I'm not every experienced when it comes to Salesforce reporting.  At first I tried a straightforward formula of (ClosedDate - OpenDate) *24 to find the resolution time for each case, but the issue with this is that it doesn't take my business hours into account, so the average resolution time is much higher than it should be because the report includes evenings and weekends in the total.

I considered whether I should use a Cases with Milestones report instead, as my company's business hours are included in the Entitlement Process and the milestones do accurately track against the business hours, but I can't seem to achieve it through this either.

This must be a fairly common use case so I suspect I'm doing something wrong, can somebody please point me in the right direction?