You also need to make the bottom corners into a rounded shape to not have long points sticking out on the floor into the room. At the bottom corners you measure up the drop of the bed from the side and from the bottom and mark the point and use it as your diameter for a half circle.
When you measure the bed you should do it with any sheets and blankets on it as they can make a difference in the drop length.
Dust ruffles can be very tailored with pleats all around or just corner and center pleats.