Are you jumping from a trot or canter? If you are jumping from the trot, I would put a trot pole about 7 to 9' in front of the fence, which will help guide you to the fence correctly. At both gaits, work on keeping a steady rhythm to the horses stride coming in, this will help make the timing of the jump more consistent. Either way, what upnover said is correct, and I couldn't have said it much better myself. What I think about is staying still with my upper body, moving my hands forward on the neck, and staying soft in my hips and squatting. If I stay still and soft, the horse will close my hip angle for me at the correct time. No getting left behind or jumping early
What hflmusicislife said is also correct about finding the spot 3 strides out. Right now, you don't need to worry much about it, let your horse get you to the correct location and let the horse make any corrections needed while you are learning. But if you learn to see when you are 3 strides out, you will eventually get to where you can help adjust the horses stride as well.