Email metrics on tags

Hello, my boss would like a metric (dashboard report) to show how he is doing against his target to email 5 contacts per day, contacts who have a long term nurture tags assigned to them. I have the saved search on his dashboard showing the contacts with this tag, but how would I add a metric to calculate if he is meeting goal to email 5 of them per day? OR, once he does email a contact and “send & record” with Outlook IS application, is there a way for the tag to be removed so that the report dwindles in size as he send emails? Thank you!