Attrition Formula??

Hi Seniors,
I have been in HR for couple of years now & all of a sudden I had gone blank in calculating attrition.
Kindly check the below formula & correct me if I'm wrong.
{Employees Left for the month/ [(Opening Head Count+ closing head count)]/2}*100
This formula is for calculating monthly attrition I suppose.
Taking the Average of every month gives Annual Attrition ?????

Dear Pradeep,
Employee Left 10
No. of employee as on 01st Jun 2012 200
No. of employees joined 3
No. of employees as on 30th Jun 2012 198
Attrition Rate 5.03%
For further clarifiaction feel free to write or discuss.