Average Leads Created per month/year/quarter

Average Leads Created per month/year/quarter

HI,

I have recently integrated Zoho CRM with Zoho Reports for Advance Analytics. I created a report for Leads that shows me Leads created per week in a month say
Week1: 10
Week2: 20
Week3: 30
Week4: 40

It shows the grand total i.e. 100 Now I also need to show average leads created this month that would be (10+20+30+40)/4 = 25.

I tried using formulas but not successful. Can you please explain me how can I do that ?
Please note I'm using a user filter for time period like user can select This Month, This Year, This Quarter etc but dimension remain same as WEEK. so number of weeks can vary.

Thanks,
Hasan