To calculate staff turnover, I would use the W-2 wage statements for the most recent year.
My first step would be to sort the W-2's into meaningful groups such as full-time professional, full-time clerical, and part-time. (Perhaps you will sort your company's W-2's by department.)
For each group, I would count the number of W-2's for people that were no longer employed at the end of the year. Let's assume that 6 full-time professional, 3 full-time clerical, and 5 part-time employees had W-2's but they had left the company before the end of the year.
Next I would divide the counts of people who had left by the average number of filled positions in the company during that year. Let's assume that on average the number of filled positions during the year were 20 full-time professional, 6 full-time clerical, and 8 part-time.
The staff turnover for full-time professional would be 30% (the 6 people leaving during the year divided by the company's 20 positions). For the full-time clerical, the staff turnover rate is 3 divided by 6, for a rate of 50%. For the part-time employees, the staff turnover rate is 62.5% (5 leaving divided by 8 positions).
After computing the staff turnover, be sure to discuss the staff turnover rate with the company's management. Training new employees only to have an above average person leave after the training period is very expensive.