Show the full calculation as a written equation:
365 - 52x2=104 Wochenendtage - Urlaub - Feiertage = Arbeitstage.
Fixes "104 Wochenenden" → "52 x 2 = 104 Wochenendtage".
Interactive calculator: enter annual hours target + vacation days,
shows actual billable hours per working day after subtracting weekends,
vacation, and public holidays. Makes the absurdity of 2400h targets visible.