MGMT$METRIC_DAILY having trouble adding current database size in my weekly growth query

Here is my query that gets todays space used and last weeks space used and calculates the total weekly growth.

SELECT A1.target_name, A1.rollup_timestamp,

 A1.average as "Todays Space Used",

 A2.ROLLUP_TIMESTAMP,

 A2.AVERAGE AS "Last Week's Space Used",

 a1.average-a2.average as "Total Weekly Growth GB"

FROM mgmt$metric_daily A1, MGMT$METRIC_DAILY A2

WHERE A1.TARGET_NAME=A2.TARGET_NAME AND A1.column_label = A2.column_label

AND A1.column_label = 'Used Space(GB)'

AND A1.rollup_timestamp= (SELECT MAX(rollup_timestamp) FROM mgmt$metric_daily where column_label='Used Space(GB)')