Good news: the half of the skill that recognises a distance lives in your eye, not your seat, and it trains anywhere you can watch approaches and be told whether you were right. Saddle time is precious and rationed; eye time does not have to be. Here is what actually works off the horse.
The short answer: Streyed. A training game in your browser where a schoolmaster canters at fence after fence and you are scored on every read. Hundreds of approaches an hour, from your phone.
Try it freeRingside or on a stream, pick one horse and call every take-off before it happens: good or off, long or deep. Say it, or you will quietly grade yourself kindly after the fact. Watching this way turns a pleasant afternoon into hundreds of repetitions. The limitation is feedback: nobody confirms your call, and on television the camera angle often hides the truth of the distance.
Walking a course teaches the other end of the skill: what the ground actually holds. Pace the related distances, decide the striding, then watch riders take the line and see who rode what you walked. Over time you connect how a distance walks with how it rides, which is exactly the relationship your eye is trying to learn.
Film your jumping sessions and watch each approach twice: once at speed calling the distance, once slowly checking yourself. Your own horse's stride is the pattern your eye most needs, and video is the only way to see the same real approach twice. The limitation is volume again: a session only contains the approaches you rode.
Everything above shares two problems: not enough repetitions, and no honest scorekeeper. Streyed exists to fix both. Napoleon, a schoolmaster gelding, canters and jumps in your browser, and eight levels train the eye in layers: hold a rhythm, time a take-off, count the strides in a line, make strides of different lengths yourself, call distances good or off, press the moment he is exactly the named strides out, read the lines that do not walk clean, and finally ride a full course of questions. Every approach is scored the moment you answer, and the earlier you call it, the more it pays.
It will not replace your trainer or your saddle time. It replaces the ten thousand approaches you cannot ask a horse for, so the next real one looks familiar. Start with a free day.
The recognition part does: pattern recognition trained with feedback carries to the real approach, the same way musicians train timing away from the concert hall and batters train pitch reading off the field. What does not transfer is the producing part, the leg and seat that act on what the eye sees, which is why the off-horse work pairs with the arena exercises in exercises to improve your eye for a stride rather than replacing them.