Probably the easiest way to generate a report like that would be to use custom properties to help you filter the interfaces you want included in the report. It will take a little bit of work to get the custom property assigned to the appropriate interfaces but once you have that set up, that should get you the report you want. Custom properties can be assigned to nodes, interfaces or volumes. There are several predefined properties that you can utilize or you have the ability to create your own from scratch.
On your Orion server, go to your start button | All Programs | SolarWinds Orion | Grouping and Access Control | Custom Property Editor. When the custom property editor opens, you can click the Edit Interfaces option. This will list all of the interfaces in your NPM inventory. In my example, I used one of the default custom properties "Comments" and assigned a property called WAN.
Once you have the custom property assigned to the appropriate WAN interfaces, you can use that custom property in report writer to filter the interfaces included in the report. One of the out of the box reports under Historical Traffic Reports is Average and Peak Traffic Rates - Last Month. As you probably already know, that included every interface in your inventory. With the WAN custom property assigned to your WAN interfaces, you can now edit this report to only include the interfaces where the Comments custom property equals WAN. Open the report in report writer and click the Filter Results tab. Click the box with the three periods in it and select "Add a new elementary condition." Click the first asterisk and select Interfaces | Custom Properties | Comments (or whatever custom property you set up.) Click the second asterisk and type in the value that's equal to your custom property you assigned to your WAN interfaces (in my example, WAN.)
With that filter applied, if you preview the report you should only see the interfaces which have the custom property assigned.
If you opened the default report, you can now select File and Save As to save this as a new report. Hope this helps!
Thank you for responding! I actually have done exactly what you suggested, but the problem is that this doesn't give me the 95th percentile average utilization in percentage...