I am sorry about being so analytical but there is a fundamental flaw in the following equation which makes the management of this KPI very suspect. While workforces and number of positions change over time this description infers that you can have an equal representation of staff turnover by having a simple equation of the number of departures over a period of time over the current employee base.
The simple question “What period of time does this represent?” allows for different interpertations as to the validity of this metric. If you have a 12 month departure over a single month number of staff member employed does not take in to account seasonal or changing workforces. I propose that an Annual Staff trunover metric be used. This could be where you use the number of departures over a 12 month period over the sum number of positions open ofer the same 12 month period.