There are many good write-ups on this one so i will not duplicate efforts.

Check out this link, a well written guide;

After firing up OpenVAS and getting a scan going you will realize that the default QoD is 70%,to get confirmed vulnerabilities use QoD 100% but to get a report of probable vulnerabilities use QoD 60%

under Tasks, choose the name of the scan to edit it then click on the blue wrench icon and change the min QoD to x%

another quick resource for tweaking the OpenVAS environment