You have to play with it a little to determine what best fits your horse. If you can figure out your horse's trotting stride over poles, it will give you a good place to start. If your horse's trot stride is 4', that would give you the distance from the first ground pole to the first crossrail. 8' later would roughly give you a bounce stride at the canter. It you added a vertical after that, you'd factor in another 3' on each end for landing and takeoff at the canter which would be 14' between the crossrail and the vertical. It's easiest if you have a person on the ground who can adjust the jumps a bit if your horse is struggling to meet the distances. Most horse's trot stride is around 4'6". I just used 4' since your horse is a pony.
Another thing you can do is figure out his trot stride using ground poles, and then figure out the canter stride the same way. Then add in roughly 3' whenever takeoff or landing in the canter is involved. Hope that helps.