The best position for a roof window is usually to one side of the desk and a little in front, high enough that the sun does not strike the glass of the monitor. Directly behind the screen throws a bright patch into your eyes. Directly behind you puts a reflection on the display. Slightly to the side of the work gives even light across the paper and keyboard and leaves the screen alone.
Loft rooms add a puzzle of their own. The ceiling slopes, so the unit often sits lower than people expect, and the part of the room with full standing height may not be the part with the desk. A VELUX fitted over the desk area still helps, because roof windows are made to sit within the slope, with the head close to the ridge side of the room.
Evenings call for a lamp, but a daylight source overhead changes how long you can work before reaching for a switch. A rooflight above and ahead of the desk lights the paper, the keyboard and your face from a high angle, much as the sky itself does, and the room stops feeling like a cupboard with a screen in it.
A blind matters too. Strong summer sun through a south slope can heat the room and wash out the screen. We raise the blind question at the survey, since the fixings and the cavity behind the frame are easier to prepare before the plaster goes on.
Glare on a screen is worse than gloom around it, so the chair is where we begin.