Hi, i dont. Nobody always finds the perfect distance. I just try to work with poles at home and for example in a line 21m i try to do it with 5 strides, then 6, 4,7 etc! Thats how i learned to control my distances better. Give it a try, maybe it works for you, as it did for me! xx
I think the most important is not "finding" a good distance but to have the good canter. If you have the good canter and your reins in connection with your horse's mouth, it will be a lot easier to have the good distance.
At home you can do some flat work to prepare jumping session by making sure you can make your horse do smaller strides and larger ones. Pole work can also helps with that.
Hope this helps!
Practice first with changing the number of strides between two poles on the ground until you get so bored with it that it becomes easy 😁 When you know how big the strides of your horse are it will get easier to see the distance to a jump.
Know how big or small your horses stride is. Practice reading into the jump how many of your horses strides it will take. Pick out your "take off point" as soon as possible. If you're coming off of another jump look into your next jump rather than down.