Struggling with Attrition Calculations: Can You Help Me Verify This Formula?

Hi Seniors, I have been in HR for a couple of years now, and all of a sudden, I have gone blank in calculating attrition. Kindly check the formula below and correct me if I'm wrong.

Attrition Calculation Formula

{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?

Thank you.

Dear Pradeep,

Employee Statistics for June 2012

- Employee Left: 10
- Number of employees as of 01st June 2012: 200
- Number of employees joined: 3
- Number of employees as of 30th June 2012: 198
- Attrition Rate: 5.03%

For further clarification, feel free to write or discuss.

Regards