Practice practice practice. Set up ground poles or whatever you and your horse are really comfortable jumping that's easy and just keep jumping and counting down from three. As you approach the jump, when you think you're three strides away, count, 3,2,1, wrong or right, do it 1000 times and you will be able to count from 10 strides away before you know it.

Another handy thing is the set up ground poles at a standard 8 or so strides away. And canter through them, first get the 8 strides, then shorten your canter and get 9 strides, and then lengthen and get 7. See how far you can go either side and you'll establish an eye to see how much ground you cover to help see distances. Remember to keep counting and don't worry if you take a while to get it.