If you're counting back only working days (Mon thru Fri), you should set up Case functions that count back a specified number of days depending on the day of the week of your original date. From what I understand, you're trying to set up at least three different date fields based on your original date. Try these calculations - be sure to set the calcualtion result to Date.
Date Less 8 = Case(DayofWeek(Original Date)=1,Original Date-11,DayofWeek(Original Date)=2,Original Date-12,DayofWeek(Original Date)=3,Original Date-12,DayofWeek(Original Date)=4,Original Date-12,DayofWeek(Original Date)=5,Original Date-10,DayofWeek(Original Date)=6,Original Date-10,DayofWeek(Original Date)=7,Original Date-10)
4 working days prior to that =
Case(DayofWeek(Original Date)=1,Original Date-17,DayofWeek(Original Date)=2,Original Date-18,DayofWeek(Original Date)=3,Original Date-18,DayofWeek(Original Date)=4,Original Date-16,DayofWeek(Original Date)=5,Original Date-16,DayofWeek(Original Date)=6,Original Date-16,DayofWeek(Original Date)=7,Original Date-16)
6 working days prior to that =
Case(DayofWeek(Original Date)=1,Original Date-25,DayofWeek(Original Date)=2,Original Date-26,DayofWeek(Original Date)=3,Original Date-26,DayofWeek(Original Date)=4,Original Date-26,DayofWeek(Original Date)=5,Original Date-24,DayofWeek(Original Date)=6,Original Date-24,DayofWeek(Original Date)=7,Original Date-24)
This solution doesn't account for holidays. Good luck.