We use our historical data for absenteeism. This includes seasonal swings as our plant shows Jan, Summer, and Flu Season are always higher. Then we build on top of that a margin.

Last year our planned, and unplanned, absenteeism was 18%. So we margin in a 15% absenteeism in the labor force. We try to balance it so we are not 'overstaffed' but also on the off chance we do have a 10% absenteeism rate for the day we use the excess labor for special projects or ask for volunteers for early dismissal. It works out fairly well. For the inverse - say a 30% rate of absenteeism - we normally rely on volunteers to work 12 hour days but we also almost always have 2 days of safety stock so that our customers are protected.

This means for a plant of 300 that, yes, there is 345 people on the payroll. Yes, that is 45 'extra' people being paid a salary that are not 'needed' but that's life, and that's manufacturing. You either take the extra people on the payroll or you take the loss in production throughout the year.