Agree with kaykat.
An eye for distance is really based on a feel for striding more than anything else.
To expand on the exercise she mentioned, place two poles 90' apart - that's a standard 7 stride line. Canter it in your normal jumping pace until you can do it in a steady even pace with your horse centering his or her stride over the pole. Once you've mastered medium pace and the 7, do it in a forward 6 then a waiting 5. The advanced version of the exercise is to have someone call either 5, 6 or to you while you're getting ready to approach the exercise, and you adjust stride and get that striding.
This exercise also helps your horse's adjustability and eye.
A follow up exercies is to place a placement rail one stride out of a single fence. If you get the correct distance to the pole, you'll be correct to the fence. And hey, you've already practiced riding to the pole, right?
Another great exercise with a single fence is to ride in your shortened canter and ride for a deep spot/small arc, medium canter/mid spot mid arc and then forward canter/long spot/long, flat arc.