It is definitely something you have to get used to over time. With experience you will start being able to see distances to the jumps easily. Counting the strides, even out loud, is helpful. Make sure the horse has a steady pace up to the fence and isn't rushing or slowing down too much. If your horse is more experienced than you are, if their pace is steady you can often trust them to find the distance themselves. It just depends.
A good distance would be when a horses strides are completely even to a fence. This can mean the distance is long, short or perfect but the length of the stride is even to compensate the distance the rider saw. Honestly training the eye is one of the hardest things and it’s very difficult to teach someone cause one day it suddenly just clicks. I would work on. A lot of pole exercises and counting 3-2-1 before the fence, and getting that perfect. Then once you’ve done that, start adding 4-3-2-1. Even counting it out loud helps. As a kid I would find lines or cracks on the ground and would count my own people strides to it (lengthening or shorting as necessary). That helps on the horse a lot later on too. But time will be your best friend on this one. Hope this helps a little!