What's the easiest way to get (Unique) Open Rates for a group of single email campaigns?

Not sure if the campaign summary report always used total opens or if that changed at some point, but I want to get unique opens or unique open rates (unique opens / delivered) by campaign to power some offline reporting. I have access to standard and custom reports, but no Advanced Analytics.

Any ideas on how to make this easy? The Total Opens / Delivered can spike above 100% sometimes.