Calculate Rolling Sickness Rate From Cumulative DataSet

Step 1) Create a proper Star-Schema model based on the data you have

Since you have only 2 tables here you need to create dims and fact via PowerQuery. First create Employee and Date dimensions and create a proper FactSickness table to allocate your sickness data on a daily grain.

Step 2) Create required measures

Available FTE (Month) = 
VAR mStart = MAX ( DimDate[MonthStart] )
VAR mEnd   = EOMONTH ( mStart, 0 )
RETURN
CALCULATE (
    SUM ( FactEmployee[FTE] ),
    FILTER (
        FactEmployee,
        FactEmployee[Start Date] <= mEnd
            && ( ISBLANK ( FactEmployee[Termination Date] )
                 || FactEmployee[Termination Date] >= mStart )
    )
)

Available FTE (Rolling 12M) = 
CALCULATE (
    [Available FTE (Month)],
    DATESINPERIOD ( DimDate[MonthStart], MAX ( DimDate[MonthStart] ), -12, MONTH )
)

Sickness Lost (Rolling 12M) = 
CALCULATE (
    SUM ( FactSickness[DailyFTE_Lost] ),
    DATESINPERIOD ( DimDate[Date], MAX ( DimDate[Date] ), -12, MONTH )
)

Sickness Rate % (Rolling 12M) = 
DIVIDE ( [Sickness Lost (Rolling 12M)], [Available FTE (Rolling 12M)] )

Step 3) Create your visual using correct field values

Please fint the attached .pbix file for end-to-end solution.