How to reduce a Test Automation Deficit

For some time, I've been wondering how on earth you make sure a Legacy application has a full suite of Automated Tests.  In my place of work, we follow Scrum as our Development Methodoligy.  I was always led to believe that you should create Automated Tests for the Stories the Team accepted in the given Sprint.

However, my colleague has sent me the attached Link, which shed's some light on how to reduce your Test Automation deficit.

How to reduce a Test Automation Deficit