To report on current disk utilization percentage I'm using:
SELECT qd.origin,qd.robot,qd.qos, qd.source,qd.target,qs.samplevalue,qs.sampletime
FROM S_QOS_SNAPSHOT qs
JOIN S_QOS_DATA qd on qs.table_id = qd.table_id
where qos = 'QOS_DISK_USAGE_PERC'
order by qs.samplevalue