Always over the jump. Keep a good canter and ensure the last stride has the same rythm as the previous ones
The better quality canter you have, the more easily distances will come. If your horse does not have impulsion and rhythm in the canter, they will be more likely to set up for a bad distance, and are less likely to try to correct it themselves, since they are less likely to be actively engaged with the exercise.
If you can develop an uphill, straight, forward canter, and maintain it with relatively little effort on circles, serpentines, over poles, etc. you will be in good shape for jumping.
As far as the rider is concerned, I have found that less is more. Your job is to sit up, support with leg so that you are forward to the fence, and let the horse decide where you take off. Once you get to know your horse a bit more, and develop your eye, you will start to be able to see your distances a bit more, and then you can begin to adjust your horses stride leading up to the jump. However, make your adjustments minimal, the less you interfere with your horse, and the more you pay attention to keeping a forward canter and upright position, the better off you will be from any takeoff spot.
Keep a consistent rhythm.