The premigration percentage will not be updated until you select the Update push button on the Update window or the Add push button on the Add window.
While you have just selected 'OK' in the Advanced Feature dialog, the premigration percentage is not activated until you select 'Add' or 'Update' in the 'Add space management', or the 'Update space management' dialog.
TSM waits until you select 'Add' or 'Update' to apply your premigration percentage to the system.
When you have changed the space management attribute, select 'Add' or 'Update', and then the system will apply your new premigration percentage.