I break tablet reading distances into three main categories—Bed, Knee, and Breakfast—and define the categories by generic use case:
• Bed (Close to face): Reading a novel on your stomach, lying in bed with the iPad propped up on a pillow.
• Knee (Medium distance from face): Sitting on the couch or perhaps the Eurostar on your way to Paris, the iPad on your knee, catching up on Instapaper.
• Breakfast (Far from face): The iPad, propped up by the Apple case at a comfortable angle, behind your breakfast coffee and bagel, allowing for handsfree news reading as you wipe cream cheese from the corner of your mouth.
So: distances near, medium, and far.