I’ll have a crack at answering this…
To determine a figure to use in the planning process you will need to analyse historical data from your WFM system or ACD. What you need to measure is the Actual FTE Delivered vs Actual Required FTE and compare that to the Service Level achieved.
This can only be measured after the day has passed and an accumulation of historical data will give you the figure that should be used for planning purposes. You can either select days where your actual SL was on or close to target or plot all ‘normal’ results on a graph and use that to determine what the schedule inefficiency figure will be to achieve SL target.
You should find that to achieve target your Actual Required FTE figure will be higher than the Actual FTE delivered and the % difference between the two is what you should use in the planning process.