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!