We begin by sitting where you sit. The desk position tells us which way you face, where glare would bite and how much daylight has to cover. A window in the slope behind and above the desk gives even light without a reflection on the screen.
Height matters more than width
A sloping ceiling pulls the roof window low in the room, and a low window needs a decision about headroom. At the desk you are seated, so a window that begins at head height when standing may still sit well above your eyes when you are on a chair. We take the measurement from the seated position, and the quote shows the height of the lower edge.
The same thinking applies to a bed under the eaves, where the slope of the ceiling can hide the best position until someone lies down to test it. The pillow end is the spot, and the window goes where the first daylight of the morning falls on it.
Keeping the room cool
A south-facing roof window can warm a small room in summer. A blind on the inside, or an external awning kit, controls that. We mention this at the survey, because choosing the blind alongside the window is simpler than adding one later.
A home office is also a room where noise matters, and a Whitwick spare room next to a busy road is no exception. Rain on a roof window is audible, and a laminated outer pane softens it. Your survey is the moment to say if that matters to you.
Seated, not standing, is the height that counts when a desk is the spot.