Networking Days based on Slicer or values in a table

I think you are close to the solution.

Reference:

https://www.daxpatterns.com/events-in-progress/

Networking Days =
VAR CalendarStartDate = MIN(DateTable[Date])
VAR CalendarEndDate = MAX(DateTable[Date])
VAR Days =
    SUMX(
        FILTER(
            'Staff Absences',
            (
                ISBLANK('Staff Absences'[Absence End Date])
                    && 'Staff Absences'[Absence Start Date] <= CalendarEndDate
            )
            || (
                'Staff Absences'[Absence End Date] >= CalendarStartDate
                    && 'Staff Absences'[Absence Start Date] <= CalendarEndDate
            )
        ),
        VAR StartDate = MAX(CalendarStartDate, 'Staff Absences'[Absence Start Date])
        VAR EndDate =
            IF(
                ISBLANK('Staff Absences'[Absence End Date]),
                CalendarEndDate,
                MIN(CalendarEndDate, 'Staff Absences'[Absence End Date])
            )
        RETURN
            NETWORKDAYS(StartDate, EndDate, 1, 'Bank Holidays')
    )
RETURN
    IF(ISBLANK(Days), 0, Days)

See if this works for you, then we can tweak for little more simpler Dax code ...