Hii,
You can do it 2 ways.In my view attrition is 2 types,monthly attrition & annual attirtion.However,you will be always more interested for annual attrition.For that you may annualize each months actual attrition to reach at the avg. attrition at the end of the year.This will give you the correct picture & through out the year,you can have an eye on the end figure of Annual attrition.Its just like run rate per over In Cricket so that at end of 50 ovrs how much will be your total score.
One formula we may use is =
(Total employees resigned/avg. head count)*(12/no. of months you have completed in that financial year)*100
To explain,suppose after end of 4 months,total resignation no. is 200.Total head count in these 4 months being,3000,3050,3075,3065.
So the annualized attrition will be (200/3047)*(1/42)*100=19%.
That means your attrition after 4 months is & after 12 months will be 19 %..Next month,you can check considering 5 months & find if it has increased or decreased.