Andrew
Very interesting topic. I manage around 150 - 200 temps in my business of a workforce of 500. The attrition rate varies dependant on your geographic location, local job market, local rates of pay, the ratio of temps to perms that you have and the potential of being employed on a full time basis.
When you have established these facts I would enter into negotiations with your prefered agency about having an absence pool. I have negotiated this with the agency that I have sourced and have set a 3% of my total headcount pool supplied daily by the agency for my absence.
I have an agreement that the agency cover the cost of the first two hours of the shift for the pool if there is no requirement for them that day at their cost, and then send them home.If I use them that shift I cover the full 8 hour costs. The advantage to you will be that all your absence is covered and you won't require overtime cost of your FTEs. The attrition rate of the agency is immaterial as you won't pay for any of the labour which is absent.
When you work out what your attrition percentage is and the above criteria, set up a KPI review weekly with the agency on absence and give them the targets. After all you are the customer.
Christian Francis