You can do this either per aggregate or globally.
To change on one aggregate, in the classic DFM web gui, browse to the list of aggregates, click the aggregate you want to change the threshold for, then in the lower right click "Edit Settings". Fill in a % value in the appropriate box for the parameter you want to change and click "Update".
To change the global value, go to Setup -> Options - Default Thresholds. Change the value for nearly full and "Update".
--erik
