Average Event Hours per Calendar Week

I'm attempting to create a report showing the average hours spent in meetings per calendar week. It's an Activities with Accounts report, filtered to the current calendar year. The hours value is a formula field converting the standard Duration (Minutes) field into an hour numeric value. It's grouped by Date,  Grouped by Calendar Week and the Duration (Hours) is Sum = TRUE (Summary "Sum" is checked).  

Yes, on December 31st of this year, I could make a simple summary formula dividing all Event Hours from the past year by 52 weeks. However, I'd like to be able to see on a rolling basis, as weeks pass throughout the year, what the running average is: Grand total Duration (Hours) SUM / # of weeks that have happened so far this year (grouping count). For example, it's the 5th week of the year. Week 1: 2 hrs of meetings, week 2: 3 hrs, week 3: 4 hrs, week 4: 5 hrs, week 5: 6 hrs. 

20 hrs / 5 weeks = avg 4 hrs per week so far this year. 

I've built a Summary Formula Column that is "valid", but always returns 1, regardless of the report date range. Displaying at Grand Total only.  

DURATIONHOURS:SUM / PARENTGROUPVAL(DURATIONHOURS:SUM, GRAND_SUMMARY)

Does anyone have a similar example I could adapt where you're using a grouping count?  

Thank you!