Is it possible in the dashboard to report a value which is a custom calculation between fields?
I have an event called "banner_impression" and another one called "banner_click"
I would like to calculate which is the percentage of "banner_click" events over "banner_impression" events.
Any way to report such calculated value in a dashboard?
Nope. Sadly calculated metrics are not a feature in any version of Google Analytics. I hear Adobe Analytics has it, but that's an enterprise-level paid solution.
You'll have to export the data out of Google Analytics, open it in Excel, and then divide your clicks by your impressions to include it in any sort of dashboard. Excel plugins are available that can help with this, and they include Analytics Edge, Next Analytics, and the Napkyn Reporting Manger.
There are also dashboarding applications like Klipfolio and Mixpanel that can allow you to suck in data from various sources (incl. Google Analytics) and present your data in a much more appealing way.