What a field hour actually costs you, once payroll taxes, comp, benefits and the hours you pay for but cannot bill are all carried.
The paid hours you cannot bill are the whole point. This spreads wage and burden across all 2,080 paid hours, including the paid time off, because you pay for those hours and get no billable output from them. A calculation that spreads the wage across billable hours only, without carrying the time-off hours, returns a lower number, and lower is the wrong direction to be wrong in. A standard year of 2,080 paid hours and an 8-hour day are assumed. The overtime figure is the cost of that hour itself and does not re-spread annual benefits, which are already carried in the billable-hour rate.