Standard annual Agency hours – for fee calculations

I am negotiating the fee with my ad agency and am struggling with the standard hours per year used for calculations. They are using 1680 but I am unsure? Answers varied from 1600-1800 hours also bearing in mind formulas for calculating such as below. An organisation that considers 40 hours per week to be full time would have roughly 2080 hours in a year. Some are holiday hours, some are leave ... ................
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