I think its double. With net curtains I tried 2.5 to 3 times the width and the fullness was too much. With heavy curtains 2.5 times would work IMHO . I suppose the thickness of the material would also have a bearing. Hickeys would probably be a good place to seek confirmation.
As with all material when they settle down they will appear flatter. Saw an article years ago where it mentioned that you should tie them in place for the first few days to get them adopt the right shape.