Percent of customers with an opportunity


I'm looking for help, please, creating a 'Percent of "Prospect" Accounts with an Open Opportunity' report. 

A couple of variables I need to account for.

1. A given territory has anywhere from 20 to 100 accounts

2. I'm trying to generate a report that would only look at 'Prospect' accounts

3. I'm using the 'Tag' field to denote Prospect accounts

Example: A territory with 100 general accounts. Within the 100 accounts there are are 20 Prospects. Of the 20 Prospects, 2 have an Open Opportunity. The desired result in this example would be 10% (2 of 20).

Thanks in advance.