Incremental or Differential backups? Incremental uses least space (one copy of files that are unchanged and only one copy of each subsequent change).
There are examples of both on the contextual Help on the Fast Backup page (F1 then scroll to end). Note that any built-in units (which are all calendar-based) above 1 week (i.e. using %DAYOFWEEK%) will hit anomalies because no higher value has a constant cycle in the ISO calendar. You'd need to either
a) grit your teeth and update your Full backup regularly, as per the small-but-constant-duration Variables available (note that the Full backup would only be refreshed with updated/new files since last-Full, not entirely re-copied/replaced)
b) live with the anomalies if using larger cycling values
c) create and use (via installable scripting) a custom Variable that has a stand-alone (not referencing any external event) range/reset-point
There are examples of both on the contextual Help on the Fast Backup page (F1 then scroll to end). Note that any built-in units (which are all calendar-based) above 1 week (i.e. using %DAYOFWEEK%) will hit anomalies because no higher value has a constant cycle in the ISO calendar. You'd need to either
a) grit your teeth and update your Full backup regularly, as per the small-but-constant-duration Variables available (note that the Full backup would only be refreshed with updated/new files since last-Full, not entirely re-copied/replaced)
b) live with the anomalies if using larger cycling values
c) create and use (via installable scripting) a custom Variable that has a stand-alone (not referencing any external event) range/reset-point