The amount of natural daylight required really depends on the type of room that the windows are being install in. As a guide work on approximately 10-12% of the total floor area for a standard habitable room, but if the room has a specific requirement for extra daylight then a ratio of 15-20% will work very well. It is worth noting that sometimes 2 smaller windows will work better than 1 single window as the spread of light can be more balanced. It is also worth considering what the overall appearance of the windows in the roof will look like.
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