For over kitchen islands and work spaces I use a very simple method to figure this out – Just measure from the ceiling to the eyebrow height of the tallest person in the house! This way the lights are high enough so they won’t interphere with normal conversation, but still low enough that they remain visually “connected” to the work surface.
A general recommended drop height for over tables is 30″ – 36″ and using the above guideline the fixtues end up within that range for an average sized person. I’m 5’11″ tall and the above formula works out to about 33″ over the counter top.
Some are inclined to hand their lights fairly high, but I favor the above method since you want the lights to be both seen in the room and to work efficiently for the work surface.