Hi, the help file has extensive details and examples on using Fast Backup. However, there is no variable with a 15 day cycle so I don't think it's possible with SyncBackSE (with Pro you can create your own custom variables using scripting, but although you've posted in the SyncBackPro forum you do state you're using SyncBackSE).
If it's acceptable you could set it to do a full backup when the current day is 15?
If it's acceptable you could set it to do a full backup when the current day is 15?