There are 3 rules that can be implemented for the purpose of days-off calculation:
Rule 1 - default, where day off is one for which there are no flights, with appropriate number of free nights
Rule 2 - day off is one for which there are no flights (Rule 1) and no duties defined in the roster
Rule 3 - day off is one for which there are no flights (Rule 1) and that day is marked as off in the roster
These options will influence the way days-off are showing on FTL Sheet and Crew Duties screen.
In orded to choose any other rule but Rule 1, you should contact Leon Software Support.