I don't think there is an exact angle your body should be at. It is more of a "feel" you get the hang of by following your horse.
Watch grand prix riders and you will see they have various two points all depending on the jump and how their horse jumps it.
To get more of a natural feel, do this with a trainer of course. Close your eyes as you take off and let your body follow your horse. It is a more simple exercise in gridlines.