Hello - Our HR Customers typically report 50% to 80% case deflection when they make quality knowledge articles available to employees through the Employee Service Center portal.  There are good webinars and case studies from Magellan Health, Christus Health, and Hitachi Ventura on the ServiceNow corporate website that you can access.  Magellan Health reports 80% case deflection and 100% of cases come through their portal.  Like many customers, they discontinued email as a channel for raising cases.  

In terms of measuring case deflection, the easiest way is if you know how many cases you were processing before releasing the portal and knowledge and then comparing.  If you were not counting cases prior to releasing the portal and knowledge, it gets trickier.  There are a lot of opinions for how to measure case deflection.  None of them are perfect, but here is the best in my opinion.  It will require some scripting in a scheduled job.  Compare employees knowledge article views to cases they raise.  If an employee views an article and subsequently does not open a case within 24 hours of viewing that article, then it counts as a case deflected because the employee answered his/her question via self-service.  

If anyone would like to submit a script for the scheduled job I've described, I think that would be widely helpful.