LOL. There are a lot of technical rules for it, but really, in the moment, you just have to see it and know it is right and that you need to ease off to make it, push for it, or hold steady. You also have to know when to get out of your horse's way and let them take the spot they need. Over time, you develop a feel and an eye for it that becomes instinctual. Even when I didn't ride for several years :(, I couldn't watch show jumping without mentally looking for all the spots.
Also, it's more than the height of the jump...if it's a wide oxer that's 2 feet tall, you need to take off differently than for a bar that's 2 feet tall.