Set up one pole, canter over it and try to judge the one stride in front of it before you go over it. Then judge two strides. Then three, then four..once you get the hang of it..
You'll learn to judge the distances and feel how much room you have.. So you can work out whether you need to shorten him up a bit more to fit an extra stride in or whether you want to push it out and have one less stride and take off a bit earlier.