My current trainer uses the expression “a quality canter”.
This is a state of being in which the horse is straight, coming through from behind and is soft to my hands. Pace is the variable here. But if you have a quality canter, you can lengthen or shorten easily.
By getting used to the elements which comprise “a quality canter” and learning what one feels like, I no longer have to go through my checklist after I land over each jump. The steps are: jump, land, balance, establish a quality canter, stay out through the turn and maintain your canter to the next jump.
As others have said – it is the canter that is the key. Is more than just pace. It is impulsion and balance and straightness. If you head to the jump with this canter, distances will show up.
I find when I am having trouble seeing spots that, inevitably, my horse is not straight/ is bulging/ or is drifting off to one side. If you are trying to see a spot that is in a straight line between your eyes and the jump, while your horse is on a lateral path to the jump, then you are looking at one distance while your actual path is another.
A good way to double check this is to lay rails about every 20’, about 4’ apart in a straight line to the jump, starting about 4 strides back.
Once you enter this “landing strip” your horse should easily canter down its center without any correction from you. – But I bet you will have to correct him because I bet he isn’t straight.
After several times over this jump, land and canter on to another one. You should be able to feel the bulge/drift/lean now because you have just had to correct it. As you do courses, keep the one “landing strip” jump as part of the course. It will be your “check up” jump to see if your horse is cantering straight on his own.