Creating calculate metric in Pivot table (Dashboard)

Hi Community,

I am trying to create a monthly summary (email sent, emails delivered, emails open, email bounced, number of clicks & Open rate) of email totals using pivot table in Dashboards. I have created group rows - Year and Month from Finished on Date on the email object using calculated field. Open rate is Number of opens in a month/number of emails sent in a month. When I am trying to create a calculated field Open Rate, it is calculating open rate for each row first and then summing up for the month. I want this to sum up both opens & sent on month first and calculate open rate later. How should I go about it?

Thanks