Rule of Thumb for Natural Room Lighting

Our 18 x 36 foot family room has two west facing windows, each 36 & 72" in size. The span between the windows is about 15 foot. These windows are it for getting natural light to the room if the small amount of light coming from two interior hallways and a kitchen entrance are discounted. Naturally the room is dark during the day.
I'm trying to minimize the work and expense to improve the room lighting. Is there a rule of thumb for lighting a room naturally as we are considering having a set of Solatubes installed. Installing another window on the west wall is not an option we want to consider.
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