You are correct that you only get to define a "day" and a "week" once. i would still likely work with a day = 8hrs and a week = 40hrs as the hours do add to 40. As long as you have set the working calendar (Change working time) correctly, it should still schedule fine. If you enter a two day duration task that starts Tuesday morning at 7:00 am, it will finish 16 working hours later.