Turnover Rate FY not Rolling 12 month

[corange]

Hi everyone, 

I need some guidance on a formula as I am relatively new to BI. 

The below is what I have used to calculated my Annual Turnover Rate: 

Employee Growth = 

VAR MaxDate = MAX('Calendar'[Date])

VAR EMPCOUNT =

CALCULATE(COUNTROWS(CALCULATETABLE(Drivers, Drivers[StartDate] <= MaxDate, ALL ('Calendar'[Date]))),

(ISBLANK(Drivers[FinishDate])|| Drivers[FinishDate] > MaxDate))

RETURN IF(ISBLANK(EMPCOUNT), 0, EMPCOUNT)

Count 12 months before =

VAR SelectDate = NEXTDAY(SAMEPERIODLASTYEAR(LASTDATE('Calendar'[Date])))

RETURN

SUMX(Drivers,

VAR DriverStartDate = [StartDate]

VAR DriverFinishDate = [FinishDate]

RETURN IF (DriverStartDate < SelectDate && OR(DriverFinishDate >= SelectDate, DriverFinishDate = BLANK ()),1,0))

Left past 12 months =

VAR selectedDate = LASTDATE('Calendar'[Date])

VAR selectedDate12MonthsBefore = NEXTDAY(SAMEPERIODLASTYEAR(selectedDate))

RETURN

SUMX(Drivers,

IF([FinishDate]>=selectedDate12MonthsBefore && [FinishDate]<=selectedDate, 1, 0))

Turnover % =

VAR averageEmployeesRolling12months = ([Employee Growth]+[Count 12 months before])/2

RETURN

[Left past 12 months]/ averageEmployeesRolling12months

I was wondering what will be the changes I need to make if I wanted to have the calculations not based on a rolling 12 months but tight to the Financial Year. 

Could you please help? 

Thanks in advance.